Fix plpython3 expected output.

I neglected this in the previous commit that updated the plpython2 output,
which I forgot to "git add" earlier.

As pointed out by Rodolfo Campero and Marko Kreen.

src/pl/plpython/expected/plpython_types_3.out

@@ -664,6 +664,9 @@ SELECT * FROM test_type_conversion_array_error();
 ERROR:  return value of function with array return type is not a Python sequence
 CONTEXT:  while creating return value
 PL/Python function "test_type_conversion_array_error"
+--
+-- Domains over arrays
+--
 CREATE DOMAIN ordered_pair_domain AS integer[] CHECK (array_length(VALUE,1)=2 AND VALUE[1] < VALUE[2]);
 CREATE FUNCTION test_type_conversion_array_domain(x ordered_pair_domain) RETURNS ordered_pair_domain AS $$
 plpy.info(x, type(x))