Fix tab completion for ALTER LANGUAGE in psql

OWNER_TO was used for the completion, which is not a supported grammar,
but OWNER TO is.

This error has been introduced by d37b816d, so backpatch down to 9.6.

Author: Alexander Lakhin
Discussion: https://postgr.es/m/7ab243e0-116d-3e44-d120-76b3df7abefd@gmail.com
Backpatch-through: 9.6

src/bin/psql/tab-complete.c

@@ -1686,7 +1686,7 @@ psql_completion(const char *text, int start, int end)
 
 	/* ALTER LANGUAGE <name> */
 	else if (Matches("ALTER", "LANGUAGE", MatchAny))
-		COMPLETE_WITH("OWNER_TO", "RENAME TO");
+		COMPLETE_WITH("OWNER TO", "RENAME TO");
 
 	/* ALTER LARGE OBJECT <oid> */
 	else if (Matches("ALTER", "LARGE", "OBJECT", MatchAny))