doc: Fix syntax in example

LANGUAGE 'plpgsql' no longer works.  The single quotes need to be
removed.

Erwin Brandstetter

doc/src/sgml/plpgsql.sgml

@@ -1708,7 +1708,7 @@ BEGIN
     RETURN;
 END
 $BODY$
-LANGUAGE 'plpgsql' ;
+LANGUAGE plpgsql;
 
 SELECT * FROM getallfoo();
 </programlisting>