[Reference] [Form types] Link to the non-deprecated form parent type

I changed all the links to the deprecated (since Symfony 2.1) `field` form type to the `form` type.

This way, there's no need to click a second time to get the documentation of the parent form type, and we are ready for the Symfony 2.3 documentation which probably remove the `field` documentation entry.

A few trailing spaces have been accidentally removed as well, but I guess that's alright not to make a second PR just for a few white spaces.