Update to use the actual script names in templates (#9)

otherwise this can be confusing, because the "start:ios" and "start:android" do not exist in the package.json scripts in the template. note: just running "yarn ios" or "yarn android" will kick off the metro bundler anyway if it's not already running :)

Author: esthor

docs/usage.md

@@ -134,13 +134,13 @@ are unfamiliar with _Android Studio_, you might be interested by
 **In another terminal:**
 
 ```console
-yarn start:ios
+yarn ios
 ```
 
 or
 
 ```console
-yarn start:android
+yarn android
 ```
 
 This commands should open up a virtual device & start React Native