message: Fix link to point to external documentation.
diff: @@ -191,7 +191,7 @@ class TfDoctestOutputChecker(doctest.OutputChecker, object):
  message = textwrap.dedent("""\n
  #############################################################
  Check the documentation
  - (go/testable-docstrings) on how to write testable docstrings.
  + (https://www.tensorflow.org/community/contribute/docs_ref) on how to write testable docstrings.
  #############################################################""")
  got.append(message)
  