Fix callback url

What does this MR do?

Fix an error on documentation for using github authentication on gitlab-ce.

When using the given callback url http(s)://${YOUR_DOMAIN}/users/auth github repsonds with an 302 because the callback url should be `http(s)://${YOUR_DOMAIN}/users/auth/github/callback`

See #57638 (closed)

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer