Update app/helpers/users_helper.rb

I didn't do the exercise at the end of chapter 7 that added the size parameter to the 'gravatar_for'.  Usually the rails tutorial will put necessary code in the body of the chapter that was in an exercise in a previous chapter if that code is really necessary.  However, adding a second parameter to gravatar_for was neglected in the subsequent chapters, but it causes the error 'wrong number of arguments (2 for 1)'.  So, I made the change here.