Merge pull request #318 from swapnilchincholkar/MONGOID-4156

[MONGOID-4156] removed persist_in_safe_mode from docs

Author: durran

source/tutorial/ruby-mongoid-tutorial.txt

@@ -3063,7 +3063,6 @@ those set in your ``config/mongoid.yml``.
   module MyApplication
     class Application < Rails::Application
       config.mongoid.logger = Logger.new($stdout, :warn)
-      config.mongoid.persist_in_safe_mode = true
     end
   end