Dynamic Form Modification example with DI

[muratbeser]

I'm sorry about my english but

http://sf.khepin.com/2012/10/user-dependent-forms-in-symfony2/ this example and comments are better then our "cookbook/form/dynamic_form_modification.rst"...

I can't explain exactly what troubled but I'm sure our guys can figure it out.

Again sorry for my bad english.

[weaverryan]

There is a PR opened by khepin on this topic actually - #1842. This needs to be merged in - it does contain some improvements overall to the document.

Thanks!