Fix passing directory argument

I got `No such file or directory` errors whenever trying to pass a directory as argument to this script. It turns out that since `Dir.chdir` is called [here](https://github.com/yardenac/eml2mbox/blob/master/eml2mbox.rb#L222), the later use at [this line](https://github.com/yardenac/eml2mbox/blob/master/eml2mbox.rb#L213) seems incorrect. Since we have already changed directory to the one specified, only `archive.mbox` should be passed to `File.new`.