Fix URL in vendor-gems.yml

[dduugg]

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cc @MikeMcQuaid ( https://github.com/Homebrew/brew/pull/13788/files#diff-abd952598a112c38ed556cd90faf0e21ae882aeab93c8f451be9d8e1ac03876aR68 )

[MikeMcQuaid]

Whoops, good catch, thanks!