Fix formatting of code block in installation-ios (#2356)

This code block appears fine on GitHub, but is malformatted on the site. This commit updates it to fit the format of other code blocks in the docs, so it should appear okay.

docs/installation-ios.md

 
 4. In Xcode, in Project Navigator (left pane), click on your project (top), then click on your *project* row (on the "project and targets list") and select the `Build Settings` tab (right pane). In the `Header Search Paths` section add `$(SRCROOT)/../node_modules/react-native-navigation/ios`. Make sure on the right to mark this new path `recursive` ([screenshots](https://facebook.github.io/react-native/docs/linking-libraries-ios.html#step-3))
 
-5. In Xcode, you will need to edit this file: `AppDelegate.m`. 
-This function:
-````````
-- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { ... }
-````````
-is the main entry point for your app. Its content must be replaced with the content of this [reference](https://github.com/wix/react-native-navigation/blob/master/example/ios/example/AppDelegate.m)
+5. In Xcode, you will need to edit this file: `AppDelegate.m`. This function is the main entry point for your app:
+
+    ```objc
+    - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions { ... }
+    ```
+
+Its content must be replaced with the content of this [reference](https://github.com/wix/react-native-navigation/blob/master/example/ios/example/AppDelegate.m)