Fix markup.

pep-0557.rst

@@ -628,7 +628,7 @@ Module level helper functions
 
   If you need to know if a class is an instance of a dataclass (and
   not a dataclass itself), then add a further check for ``not
-  isinstance(obj, type)``:
+  isinstance(obj, type)``::
 
     def is_dataclass_instance(obj):
         return is_dataclass(obj) and not isinstance(obj, type)