Update building-github-apps.md

[armandsl]

Why:

What's being changed:

Check off the following:

• All of the tests are passing.
• I have reviewed my changes in staging.
• 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]

Thanks @armandsl! This sentence needs a period at the end, so I'm going to close this out.

Next time, please use the pull request template so we can better understand your changes.