amended typo in link from htpp to http

There is a typo in the coach invitation ("htpp" rather than "http").

It stops the link from working automatically from the mail client (mine, anyway).

Please note: I've read the guidelines for contributing and my fork is installed on a machine running rails, and the code has been viewed and changed in Aptana as a Rails project. However, I'm not a rails programmer and certainly do not know how to test the mailer.

So, this pull request has NOT BEEN TESTED. Apologies if this is annoying or wastes somebody's time. I perfectly understand if this pull request is rejected.