Update Gracefully-Stopping.md

[MosesHe]

Add a missing colon in sample.

Hello there!

Make sure you've followed the Contributing guidelines before finalizing your pull request.

TL;DR:

• You're opening this PR against the current release branch
• Works on both Python 2.7 & Python 3.x
• Commits have been squashed and includes the relevant issue number
• Pull request description contains link to relevant issue or detailed notes on changes
• This must include example code demonstrating your feature or the bug being fixed
• All existing tests pass
• Your bug fix / feature has associated test coverage
• README.md is updated (if relevant)