minor #4329 ensure consistency with the note (greg0ire)

This PR was merged into the 2.3 branch.

Discussion
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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.

Commits
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8a649a6 ensure consistency with the note

book/testing.rst

@@ -612,7 +612,8 @@ and pass it a ``Link`` object::
 Forms
 ~~~~~
 
-Just like links, you select forms with the ``selectButton()`` method::
+Forms can be selected using their buttons, which can be selected with the
+``selectButton()`` method, just like links::
 
     $buttonCrawlerNode = $crawler->selectButton('submit');