Capitalize GitHub properly in contributors README

[zackkrida]

Camel/Pascal case solidarity with WordPress and GitHub! This is a microscopic PR to fix the capitalization of "GitHub" on the contributors README.

Description

How has this been tested?

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @zackkrida! In case you missed it, we'd love to have you join us in our Slack community, where we hold regularly weekly meetings open to anyone to coordinate with each other.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[ntsekouras]

Thanks @zackkrida 👍