Update form_types.rst

Very samll typo that changes a too long phrase into a decent HTML list

Resources/doc/reference/form_types.rst

@@ -229,6 +229,7 @@ collection
 ^^^^^^^^^^
 
 This bundle handle the symfony ``collection`` form type by adding:
+
 * an ``add`` button if you set the ``allow_add`` option to ``true``.
 * a ``delete`` button if you set the ``allow_delete`` option to ``true``.