Consistency fix to the Getting Started with Rails guide

This corrects the code snippet taken from the app/views/posts/new.html.erb
file on line 704.  Earlier in the guide on lines 395-398 the guide user is 
instructed to change the form_for line in the new.html.erb file to use the 
posts_path helper.  This fix updates the code snippet to be consistent with that 
earlier change.