Update book/forms.rst

Fix typo (missing ";") in source code sample for dynamic validation groups
Merge branch '2.2' of https://github.com/symfony/symfony-docs into 2.2
Collection of fixes: typos, formatting, logic, links
Change "swiftmailer" and "Swiftmailer" notation to "Swift Mailer"
Revert notation fix of "DependencyInjection component"
Merge pull request #3173 from bicpi/fixing_typos_formatting_logic_collection

Collection of fixes: Typos, Formatting, Logic, Links