update GettingStarted.md for new iOS run

Summary:Thanks for submitting a pull request! Please provide enough information so that others can review your pull request:

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the iOS run is now aligned with Android and can be run preferably from the command line.
Closes https://github.com/facebook/react-native/pull/6590

Differential Revision: D3087759

Pulled By: vjeux

fb-gh-sync-id: 9a75f960dce2f3aaa182c4e98537397e5b71dde2
shipit-source-id: 9a75f960dce2f3aaa182c4e98537397e5b71dde2

docs/GettingStarted.md

@@ -45,7 +45,7 @@ Create a React Native project:
 **To run the iOS app:**
 
 - `$ cd AwesomeProject`
-- Open `ios/AwesomeProject.xcodeproj` and hit run in Xcode.
+- `$ react-native run-ios` **OR** open `ios/AwesomeProject.xcodeproj` and hit "Run" button in Xcode
 - Open `index.ios.js` in your text editor of choice and edit some lines.
 - Hit ⌘-R in your iOS simulator to reload the app and see your change!