Fix readme syntax (heading underline length)

Author: knaperek

README.rst

@@ -111,7 +111,7 @@ you should set the LOGIN_URL option to another view and put a link in such
 view to the ``/saml2/login/`` view.
 
 Preferred Logout binding
------------------
+------------------------
 Use the following setting to choose your preferred binding for SP initiated logout requests::
 
   SAML_LOGOUT_REQUEST_PREFERRED_BINDING