Fixed small errors in README.md (#35)

[stephanvane]

worked on #35

• removed GrapeTokenAuth.configure_warden(manager) line from documentation.
• fixed typo: mount_session -> mount_sessions

[mcordell]

Thanks for the updates.

Posterity note, configure_warden removed in ae43b728c1da3a18119a98a0aa35968b1e1c5a33