minor symfony#7083 update github example (hootlex)

This PR was submitted for the 3.1 branch but it was merged into the 2.7 branch instead (closes symfony#7083).

Discussion
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update github example

Commits
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6761ecf update github example

Author: xabbuh

components/dom_crawler.rst

@@ -441,7 +441,7 @@ directly::
     $crawler = $client->request('GET', 'https://github.com/login');
 
     // select the form and fill in some values
-    $form = $crawler->selectButton('Log in')->form();
+    $form = $crawler->selectButton('Sign in')->form();
     $form['login'] = 'symfonyfan';
     $form['password'] = 'anypass';