Fix expected output of xml_2.out

Per buildfarm members snakefly, parula and prion, that reflect the
results coming from the latest versions of libxml2.

Oversight in b8da37b in the shape of an incorrect copy-paste.  The CI
was green, but it does not stress this expected output.

src/test/regress/expected/xml_2.out

@@ -30,8 +30,8 @@ SELECT pg_input_is_valid('<value>one</', 'xml');
 (1 row)
 
 SELECT message FROM pg_input_error_info('<value>one</', 'xml');
-       message          
-------------------------
+       message       
+---------------------
  invalid XML content
 (1 row)