Fix link in documentation of user roles
Fix link in documentation of user roles

The link must use the AsciiDoc format, not the Markdown format.

Signed-off-by: Edwin Kempin <d43fb6c98c97e553b671ed9a9318100f1de386e7@google.com>
Change-Id: I6d87d0ad8512e19379359f7bb0dfa1493c1255bb

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