README seems to have an error in 'Tags'

Currently the README file states:
`# Tag users`
`tag = intercom.tags.tag_users(name='blue', users=[{'email': 'test1@example.com'}])`

But this results in
`AttributeError: 'Tag' object has no attribute 'tag_users'`

Perhaps it should say:
`tag = intercom.tags.tag(name='blue', users=[{'email': 'test1@example.com'}])`
since this results in success.
