Fixed command in README for ipa distribute:hockeyapp

Hi Mattt,

In the README I changed:
`$ ipa distribute:hockeyapp -t API_TOKEN`

to:
`$ ipa distribute:hockeyapp --token API_TOKEN`

I'm pretty sure that `-t` isn't a valid option, correct me if I'm wrong.

I think most people will go by the environment variable anyway but I'm writing a gem to make setting up continuous integration environments easier and am thinking considering wrapping Shenzhen as an optional part of the process, so this may be one less chore for whoever has been doomed with the task. Here's the repo in case you were wondering -> https://github.com/adamwaite/iOS-Build-Kit.

Thanks