Fixed mispell in doc Actvity -> Activity

Summary:
Fixed misspell in react native doc
Closes https://github.com/facebook/react-native/pull/8736

Differential Revision: D3555187

fbshipit-source-id: f9c0e978d3dc7522a2fc58c4013458c97b69a397

docs/NativeModulesAndroid.md

@@ -420,16 +420,16 @@ Now you can listen to the activity's LifeCycle events by implementing the follow
 ```java
 @Override
 public void onHostResume() {
-    // Actvity `onResume`
+    // Activity `onResume`
 }
 
 @Override
 public void onHostPause() {
-    // Actvity `onPause`
+    // Activity `onPause`
 }
 
 @Override
 public void onHostDestroy() {
-    // Actvity `onDestroy`
+    // Activity `onDestroy`
 }
 ```