Correct the gh-pages docs for the v2.0 `--without` cli arg as `--except`

See 
* https://github.com/berkshelf/berkshelf/blob/v2.0.18/lib/berkshelf/cli.rb#L131-L134
* https://github.com/berkshelf/berkshelf/blob/v2.0.18/lib/berkshelf/cli.rb#L162-L165
* https://github.com/berkshelf/berkshelf/blob/v2.0.18/lib/berkshelf/cli.rb#L187-L190
* https://github.com/berkshelf/berkshelf/blob/v2.0.18/lib/berkshelf/cli.rb#L262-L265