PEP 531: Resolve uses of the default role (#3378)

pep-0531.txt

@@ -572,7 +572,7 @@ protocol definition significantly more complex, both to define and to use:
   a singleton tuple containing a reference to the object to be used as the
   result of the existence check
 * at the C layer, ``tp_exists`` implementations would return NULL to indicate
-  non-existence, and otherwise return a `PyObject *` pointer as the
+  non-existence, and otherwise return a ``PyObject *`` pointer as the
   result of the existence check
 
 Given that change, the sentinel object idiom could be rewritten as::