Update the Github repository param documentation

[opethcire316]

When browsing the documentation for how to use a Github package binding and a trigger set up for the push event, a user will see the following information
"wsk package bind /whisk.system/github myGit --param username myGitUser --param repository myGitRepo --param accessToken aaaaa1111a1a1a1a1a111111aaaaaa1111aa1a1a"

The "--param repository ..." is too vague and not clear in what needs to be entered. Some users may think that they need to enter an entire url when really it is just the Github org/repo.