docs: typo fix

[HillaShx]

Hello team! I'm implementing Authlib in an app I'm working on, thank you for your good work! While reading the docs, I realized the name GitHub was missing a T, so I'm submitting this fix for your approval.

Thank you and have a great day!

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update
• [ ] Refactor
• [x] Other: Typo in the docs

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [x] No

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• [x] You consent that the copyright of your pull request source code belongs to Authlib's author.