He meant to replace <post number> with the post number, so it should be
ruby pixanim.rb 44579303 webm
That's just the way it's usually presented, since if he had said sth like "ruby poxanim.rb post number image format" that'd be pretty confusing. By the way, if you check the commentary part of his script, you'll notice that usage syntax is written there as well. Ah, and since you probably haven't read the commentary in his script, I suggest running "gem install rubyzip" and "gem install mechanize" in cmd before the script. Those two are unmentioned in guide since it's literally one cmd line each to install them