Update on riscv -Wformat-diag string.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_handle_type_attribute):
	Update one -Wformat-diag string in warning message.

gcc/config/riscv/riscv.cc

@@ -3309,8 +3309,8 @@ riscv_handle_type_attribute (tree *node ATTRIBUTE_UNUSED, tree name, tree args,
 	      && strcmp (string, "machine"))
 	    {
 	      warning (OPT_Wattributes,
-		       "argument to %qE attribute is not %<user%>, %<supervisor%>, "
-		       "or %<machine%>", name);
+		       "argument to %qE attribute is not %<\"user\"%>, %<\"supervisor\"%>, "
+		       "or %<\"machine\"%>", name);
 	      *no_add_attrs = true;
 	    }
 	}