Fix formatting for consistency with other default values in configuration.rst

Author: myii

docs/configuration.rst

@@ -109,7 +109,7 @@ Toolbar options
 
 * ``SHOW_TOOLBAR_CALLBACK``
 
-  Default: 'debug_toolbar.middleware.show_toolbar'
+  Default: ``'debug_toolbar.middleware.show_toolbar'``
 
   This is the dotted path to a function used for determining whether the
   toolbar should show or not. The default checks are that ``DEBUG`` must be set