Fix docs parsing error in creds.py
Fix docs parsing error in creds.py

In the case of :class`MechTypes`s, the "s" at the end was causing
Sphinx to fail to parse the class markup.  This fixes that by rewording
slightly to remove the need for the trailing "s".

pythongssapi/python-gssapi,pythongssapi/python-gssapi,pythongssapi/python-gssapi