typo fixes for README (#1690)

README.md

 ```js
 Navigation.dismissAllModals();
 ```
-#### Screen Lifecycle - didDisppear() and didAppear()
+#### Screen Lifecycle - didDisappear() and didAppear()
 
-The didDisppear() and didAppear() functions are lifecycle functions that are added to the screen and run when a screen apears and disappears from the screen. To use them simply add them to your component like any other react lifecycle function:
+The didDisappear() and didAppear() functions are lifecycle functions that are added to the screen and run when a screen apears and disappears from the screen. To use them simply add them to your component like any other react lifecycle function:
 
 ```js
 class LifecycleScreen extends Component {
     this.setState({ text: 'didAppear' });
   }
 
-  didDisppear() {
-    alert('didDisppear');
+  didDisappear() {
+    alert('didDisappear');
   }
 
   componentWillUnmount() {
   }
 }
 ```
-
-