Update IntegrationWithExistingApps.md

Summary:
The name should be "MyReactNativeApp" otherwise it gives error that the HelloWorld Application is not found

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Closes https://github.com/facebook/react-native/pull/15251

Differential Revision: D5518727

Pulled By: hramos

fbshipit-source-id: d9a2eac4698927f17f25a2316bd5674bb227821e

docs/IntegrationWithExistingApps.md

@@ -684,7 +684,7 @@ public class MyReactActivity extends Activity implements DefaultHardwareBackBtnH
                 .setUseDeveloperSupport(BuildConfig.DEBUG)
                 .setInitialLifecycleState(LifecycleState.RESUMED)
                 .build();
-        mReactRootView.startReactApplication(mReactInstanceManager, "HelloWorld", null);
+        mReactRootView.startReactApplication(mReactInstanceManager, "MyReactNativeApp", null);
 
         setContentView(mReactRootView);
     }