fix typo in dry_run attribute

Fixes the following error when running `cpan-packager`:

    Found unknown argument(s) passed to 'dry_run' attribute constructor in 'Mouse::Meta::Attribute': defalut at /home/mgardner/perl5/perlbrew/perls/perl-5.16.3/lib/site_perl/5.16.3/CPAN/Packager/Script.pm line 8.