fix capitalization of GitHub

[vtbassmatt]

Website change review checklist:

• [ ] I have requested a review from the appropriate persons
• [ ] I have checked that my changes work in the preview
• [ ] I have checked that the rest of the site is still functioning in the preview
• [ ] I have checked that my content/code is consistent with existing content/code
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This pull request:

• fixes the capitalization of "GitHub"