Update ack-and-a-half.el

Replace "-" with "_" in command line, since this does not work anymore with ack 2.04 running under Perl 5.18.0.

```
invalid filter specification 'ack-and-a-half-custom-type=rst' at /usr/share/perl5/core_perl/Getopt/Long.pm line 589.
Unknown type 'ack-and-a-half-custom-type' at /usr/share/perl5/core_perl/Getopt/Long.pm line 589.
ack: Invalid option on command line
```