Update README.md line 458

Hey there.  Love the gem.

I'm new to Rails (and OO programming in general) so I can easily get hung up on a small detail that a pro wouldn't.  I was stuck for several hours on a project simply because I didn't add the block variable to the simple_fields_for method.  This is because in your documentation on line 458, your example doesn't have the block variable added either.

I had to scour the interwebs for the solution.  I stumbled across the solution and ss soon as I added that variable, BOOM!..everything worked to perfection.

Anyway, maybe this will help...maybe it won't and I'm just off my rocker.  But thanks for a great gem and thanks for taking the time to read this.