fix example in comment, need '.repos'

[takp]

Hi I think this example doesn't work, so I changed this as following.

    #  github = Github.new
    #  github.collaborators.add 'user', 'repo', 'username'

change to

    #  github = Github.new
    #  github.repos.collaborators.add 'user', 'repo', 'username'

I hope this is right. Thanks.

[coveralls]

Coverage increased (+3.26%) to 97.77% when pulling 0ba8c59 on takp:fix-comment into 458695b on peter-murach:master.

[coveralls]

Coverage increased (+3.26%) to 97.77% when pulling 0ba8c59 on takp:fix-comment into 458695b on peter-murach:master.

[coveralls]

Coverage increased (+3.26%) to 97.77% when pulling 0ba8c59 on takp:fix-comment into 458695b on peter-murach:master.

[coveralls]

Coverage increased (+0.35%) to 94.86% when pulling 0ba8c59 on takp:fix-comment into 458695b on peter-murach:master.

[piotrmurach]

Thanks!