Fix numbering on doc instructions (#5360)

Current numbering causes the line 
```In Xcode, you will need to edit this file: AppDelegate.m. This function is the main entry point for your app:```
to appear as instruction 1, not 3.

docs/docs/Installing.md

 
 2. In Xcode, in Project Navigator (left pane), click on your project (top), then click on your *target* row (on the "project and targets list", which is on the left column of the right pane) and select the `Build Phases` tab (right pane). In the `Link Binary With Libraries` section add `libReactNativeNavigation.a` ([screenshots](https://facebook.github.io/react-native/docs/linking-libraries-ios.html#step-2)).
 
-2b. If you're seeing an error message in Xcode such as:
-```
-'ReactNativeNavigation/ReactNativeNavigation.h' file not found.
-```
-You may also need to add a Header Search Path: ([screenshots](https://facebook.github.io/react-native/docs/linking-libraries-ios.html#step-3)).
-```objectivec
-$(SRCROOT)/../node_modules/react-native-navigation/lib/ios
-```
+	a. If you're seeing an error message in Xcode such as:
+	```
+	'ReactNativeNavigation/ReactNativeNavigation.h' file not found.
+	```
+	You may also need to add a Header Search Path: ([screenshots](https://facebook.github.io/react-native/docs/linking-libraries-ios.html#step-3)).
+	```objectivec
+	$(SRCROOT)/../node_modules/react-native-navigation/lib/ios
+	```
 
 3. In Xcode, you will need to edit this file: `AppDelegate.m`. This function is the main entry point for your app:
 
 	@end
 	```
 
-3a. If, in Xcode, you see the following error message in `AppDelegate.m` next to `#import "RCTBundleURLProvider.h"`:
-```
-! 'RCTBundleURLProvider.h' file not found
-```
-This is because the `React` scheme is missing from your project. You can verify this by opening the `Product` menu and the `Scheme` submenu.
-
-To make the `React` scheme available to your project, run `npm install -g react-native-git-upgrade` followed by `react-native-git-upgrade`. Once this is done, you can click back to the menu in Xcode: `Product -> Scheme -> Manage Schemes`, then click '+' to add a new scheme. From the `Target` menu, select "React", and click the checkbox to make the scheme `shared`. This should make the error disappear.
+a. If, in Xcode, you see the following error message in `AppDelegate.m` next to `#import "RCTBundleURLProvider.h"`: 
+	```
+	! 'RCTBundleURLProvider.h' file not found
+	```
+	This is because the `React` scheme is missing from your project. You can verify this by opening the `Product` menu and the `Scheme` submenu. 
+	To make the `React` scheme available to your project, run `npm install -g react-native-git-upgrade` followed by `react-native-git-upgrade`. Once this is done, you can click back to the menu in Xcode: `Product -> Scheme -> Manage Schemes`, then click '+' to add a new scheme. From the `Target` menu, select "React", and click the checkbox to make the scheme `shared`. This should make the error disappear.
 
-3b. If, in Xcode, you see the following warning message in `AppDelegate.m` next to `#import "@implementation AppDelegate"`:
-```
-Class 'AppDelegate' does not conform to protocol 'RCTBridgeDelegate'
-```
-You can remove `RCTBridgeDelegate` from this file: `AppDelegate.h`:
-```diff
-- #import <React/RCTBridgeDelegate.h>
-- @interface AppDelegate : UIResponder <UIApplicationDelegate, RCTBridgeDelegate>
-+ @interface AppDelegate : UIResponder <UIApplicationDelegate>
-	...
-```
+	b. If, in Xcode, you see the following warning message in `AppDelegate.m` next to `#import "@implementation AppDelegate"`:
+	```
+	Class 'AppDelegate' does not conform to protocol 'RCTBridgeDelegate'
+	```
+	You can remove `RCTBridgeDelegate` from this file: `AppDelegate.h`:
+	```diff
+	- #import <React/RCTBridgeDelegate.h>
+	- @interface AppDelegate : UIResponder <UIApplicationDelegate, RCTBridgeDelegate>
+	+ @interface AppDelegate : UIResponder <UIApplicationDelegate>
+		...
+	```
 
 ### Installation with CocoaPods