Fix file name where example docs are located.

doc/ext/appapi.rst

@@ -141,7 +141,7 @@ the following public API:
    * If you provide *parse_node*, it must be a function that takes a string and
      a docutils node, and it must populate the node with children parsed from
      the string.  It must then return the name of the item to be used in
-     cross-referencing and index entries.  See the :file:`ext.py` file in the
+     cross-referencing and index entries.  See the :file:`conf.py` file in the
      source for this documentation for an example.
 
    For example, if you have this call in a custom Sphinx extension::