ensure consistency with the note

The note explains that we are selecting form buttons, not forms.
Confusion ensues if we say otherwise in the introductory sentence.

Author: Grégoire Paris

book/testing.rst

@@ -614,7 +614,7 @@ and pass it a ``Link`` object::
 Forms
 ~~~~~
 
-Just like links, you select forms with the ``selectButton()`` method::
+Just like links, you select form buttons with the ``selectButton()`` method::
 
     $buttonCrawlerNode = $crawler->selectButton('submit');