Incorrect comments in _signing.py #406
Description
In _signing.py, we can read this comment:
.. note::
Assumes ``credentials`` implements the
:class:`google.auth.credentials.Signing` interface. Also assumes
``credentials`` has a ``service_account_email`` property which
identifies the credentials.
But the assumed property is signer_email and not service_account_email.
In nowhere a property called service_account_email is read from credentials, but signer_email is.