Add .google.com link in AUTHENTICATION.md.
Add .google.com link in AUTHENTICATION.md.

See https://github.com/google/mako/pull/3/commits.

PiperOrigin-RevId: 264455983
Change-Id: I7c1acf59594532db06d04d5e4acea6ddebc84bd1

google/mako,google/mako,google/mako,google/mako