Add missing import, remove trailing ":" in middleware example

Minor fixes to the external authentication example given in the
documentation.

Change-Id: I2bef7da8bf8278349fec80a513095637ea49f19a
Closes-Bug: #1308634
Co-Authored-By: Florent Flament <florent.flament-ext@cloudwatt.com>

doc/source/external-auth.rst

@@ -100,6 +100,7 @@ following the semantics below:
 .. code:: python
 
     from keystone.common import wsgi
+    from keystone import exception
 
     class MyMiddlewareAuth(wsgi.Middleware):
         def __init__(self, *args, **kwargs):
@@ -114,7 +115,7 @@ following the semantics below:
                 # Not applicable
                 return self.application
 
-            username = self.do_auth(request):
+            username = self.do_auth(request)
             if username is not None:
                 # User is authenticated
                 request.environ['REMOTE_USER'] = username