Documentation fix for how to get the session attribute "RavenRemoteUser"

request.getAttribute("RavenRemoteUser") always returned null. The RavenRemoteUser
could be extracted from request.getSession().getAttribute("..."), though. The
documentation was adjusted accordingly to state to get the attribute from the session,
not the request.
Merge pull request #2 from Jocbe/master

Documentation fix for how to get RavenRemoteUser