Update part-two.rst

I think a 'S' is missing for the "Post Model". 
I am actually following this tutorial to begin with CakePHP, and when I try my code,
with a Post.php file in my Model repo, I get an error "No PostController found blabla..."

I find more coherent to place a 's' at the end of 'Post.php' to make 'Posts.php', so CakePHP will find PostsController alone and won't ask for PostController.

See you later guys, thank you for your doc !