docs fix (#7200)

Author: stephenworsley

docs/src/user_manual/explanation/um_files_loading.rst

@@ -100,7 +100,7 @@ as follows:
     The complete details are implemented as follows:
 
     *   The conversion of fields to raw cubes is performed by the function
-        :func:`iris.fileformats.pp_rules.convert`, which is called from
+        :func:`iris.fileformats.pp_load_rules.convert`, which is called from
         :func:`iris.fileformats.pp.load_cubes` during loading.
     *   The corresponding save functionality for PP output is implemented by
         the :func:`iris.fileformats.pp.save` function.  The relevant