fix literal rendering in dictionary options docs

Fix the syntax error in the python example for dictionary options so
pygments will render it properly.

Use a code-block directive to tell pygments that the inline INI file is
an INI file, since it does not figure that out automatically.

Closes-Bug: #1413389
Change-Id: I16441739e6cbfe8ad497aa0860fbc85a4a2d8123

oslo_config/cfg.py

@@ -274,11 +274,13 @@ use the DictOpt::
 
     opts = [
         cfg.DictOpt('foo',
-                    default={}))
+                    default={})
     ]
 
 The end users can then specify the option foo in their configuration file
-as shown below::
+as shown below:
+
+.. code-block:: ini
 
     [DEFAULT]
     foo = k1:v1,k2:v2