add another answer to a troubleshooting question

[georgicodes]

This PR updates the "Creating a github app" docs. It adds in one more troubleshooting tip that solves a problem I ran into myself whilst running through these instructions.

What's being changed:

rendered

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• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[janiceilene]

@georgicodes Thanks so much for opening a PR! I'll get this triaged for review ⚡

[janiceilene]

Thanks @georgicodes! I'll get this merged in once tests are passing ⚡