docs: don't split @smallexample in multiple @groups

docs: don't split @smallexample in multiple @groups

Noticed multiple groups split in HTML documentation where example
was written in two columns:

                                                   ""
                                                   "
  (define_expand "addsi3"                          {
    [(match_operand:SI 0 "register_operand" "")      handle_add (...
     (match_operand:SI 1 "register_operand" "")      DONE;
     (match_operand:SI 2 "register_operand" "")]   }")

The change uses single @group/@endgroup to prevent such break.

gcc/ChangeLog:

	* doc/md.texi: Don't split @smallexample in multiple @groups.