Incorrect comments in _signing.py

[DiegoQueiroz]

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.