docstring: fix typo

keystoneauth1 loading had a typo in one of its docstrings and I just
couldn't help it in the end and I had to send this patch.

Change-Id: I8705fe64a04d82868c7d4f076dfffa9d31a6b899
Signed-off-by: Antoni Segura Puimedon <toni@midokura.com>

keystoneauth1/loading/conf.py

@@ -34,7 +34,7 @@ def get_common_conf_options():
     or to manipulate the options before registering them yourself.
 
     The options that are set are:
-        :auth_type: The name of the pluign to load.
+        :auth_type: The name of the plugin to load.
         :auth_section: The config file section to load options from.
 
     :returns: A list of oslo_config options.