Update error message.

From-SVN: r131087

gcc/testsuite/gnat.dg/specs/variant_part.ads

 -- { dg-do compile }
 package Variant_Part is
    type T1(b: boolean) is record
-     case (b) is              -- { dg-error "discriminant name expected" }
+     case (b) is    -- { dg-error "discriminant name may not be parenthesized" }
         when others => null;
      end case;
    end record;