Mark the "encoding" parameter to ExternalEntityParserCreate() as optional

in the docstring.

Modules/pyexpat.c

@@ -535,7 +535,7 @@ xmlparse_GetBase(xmlparseobject *self, PyObject *args)
 }
 
 static char xmlparse_ExternalEntityParserCreate__doc__[] = 
-"ExternalEntityParserCreate(context, encoding)\n\
+"ExternalEntityParserCreate(context[, encoding])\n\
 Create a parser for parsing an external entity based on the\n\
 information passed to the ExternalEntityRefHandler.";