fix generated iOS example on React Native 0.60(+)

[brodycj]

In order to build and run the React Native 0.60(+) example app on iOS, it did not seem to work without the following steps:

• cd ios
• pod install

This seems to be an issue with React Native itself.

I hope to take a look at the following ideas someday:

• update example postinstall to do this automatically
• I think React Native itself should be fixed to do this automatically

P.S. The React Native CLI seems to automate the pod install step. I think this tool needs to be fixed to automate this step on the example app. Also in case the developer removes the generated artifacts and then does yarn install again in the example app.