fix the dead links for webhook event types

[iam-veeramalla]

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

go-github/github/messages.go

Line 258 in 7520506

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

go-github/github/messages.go

Line 265 in 7520506

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