[Perl/perl5] e372b2: groups.t: Correct one syntax error which was warning

[James E Keenan]

  Branch: refs/heads/op-groups-20200703
  Home:   https://github.com/Perl/perl5
  Commit: e372b20d88728bbb080c5974f6ea404c11df7f9f
      
https://github.com/Perl/perl5/commit/e372b20d88728bbb080c5974f6ea404c11df7f9f
  Author: James E Keenan <jkee...@cpan.org>
  Date:   2020-07-03 (Fri, 03 Jul 2020)

  Changed paths:
    M t/op/groups.t

  Log Message:
  -----------
  groups.t:  Correct one syntax error which was warning

When run with warnings, this file would get:

        Scalar value @sup_group[2] better written as $sup_group[2]

Using '@' rather than '$' was probably unintentional.  Let's fix it.