Updated README.md doc.

Making it consistent with 

https://github.com/rails/activerecord-session_store/commit/4ecd3f741c746758a2de88fcbcc8729c8292cdab#lib/action_dispatch/session/active_record_store.rb
Merge pull request #14 from ghilead/patch-1

Update README to correctly state the configuration option.

[ci skip]