Fix documentation

https://login.persona.org/verify was disabled a while ago[1] and the current code is using https://verifier.login.persona.org/verify.

[1] https://mail.mozilla.org/pipermail/persona-notices/2012/000002.html
Merge pull request #87 from jerrykan/fixdoco

Fix documentation

Thank you @jerrykan! r+, merging.