doc: Fix mistake in PL/Python documentation

Small thinko introduced by 94aceed

Reported-by: nassehk@gmail.com

Author: petere

doc/src/sgml/plpython.sgml

@@ -573,7 +573,7 @@ CREATE FUNCTION make_pair (name text, value integer)
   RETURNS named_value
 AS $$
   return ( name, value )
-  # or alternatively, as tuple: return [ name, value ]
+  # or alternatively, as list: return [ name, value ]
 $$ LANGUAGE plpythonu;
 </programlisting>