Fix xmlconcat regression test

src/test/regress/expected/xml.out

@@ -57,8 +57,8 @@ ERROR:  invalid XML content
 DETAIL:  Expected '>'
 SELECT xmlconcat('<foo/>', NULL, '<?xml version="1.1" standalone="no"?><bar/>');
              xmlconcat             
----------------------------------------------------
- <?xml version="1.1" standalone="no"?><foo/><bar/>
+-----------------------------------
+ <?xml version="1.1"?><foo/><bar/>
 (1 row)
 
 SELECT xmlelement(name element,