removed Non-ASCII dash with ASCII dash to allow pod test to pass

Which should resolve this test failure on install

#   Failed test 'POD test for blib/lib/App/gh/Command/Pull.pm'
#   at /Users/dsteinbrunner/perl5/perlbrew/perls/perl-5.18.1/lib/site_perl/5.18.1/Test/Pod.pm line 186.
Wide character in print at /Users/dsteinbrunner/perl5/perlbrew/perls/perl-5.18.1/lib/5.18.1/Test/Builder.pm line 1821.
# blib/lib/App/gh/Command/Pull.pm (40): Non-ASCII character seen before =encoding in '—'. Assuming UTF-8
# Looks like you failed 1 test of 33.
t/pod.t ....... 
Dubious, test returned 1 (wstat 256, 0x100)
Merge pull request #48 from dsteinbrunner/patch-1

removed Non-ASCII dash with ASCII dash to allow pod test to pass