fix the dead links for webhook event types

I noticed a few dead links in message.go file which may be referencing an obsolete documentation.

Line 258 in 7520506

    
           // GitHub API docs: https://docs.github.com/en/free-pro-team@latest/rest/reference/repos/hooks/#webhook-headers

Line 265 in 7520506

    
           // GitHub API docs: https://docs.github.com/en/free-pro-team@latest/rest/reference/repos/hooks/#webhook-headers

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