fix leading whitespace in Pod's synopsis

I noticed the indentation was off a bit here: https://metacpan.org/module/Devel::Gladiator

There were some funny leading whitespace characters in the synopsis, so I just replaced it all with 2 spaces

perldoc shows this now:

```perldoc
SYNOPSIS
         use Devel::Gladiator qw(walk_arena arena_ref_counts arena_table);

         my $all = walk_arena();

         foreach my $sv ( @$all ) {
             warn "live object: $sv\n";
         }

         warn arena_table(); # prints counts keyed by class
```