minor #21269 [Testing] fix phpunit.dist.xml code example (webmasterMeyers)

This PR was submitted for the 7.3 branch but it was squashed and merged into the 6.4 branch instead.

Discussion
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[Testing] fix phpunit.dist.xml code example

small update to documentation code example

Commits
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fa5a1aa [Testing] fix phpunit.dist.xml code example

Author: javiereguiluz

testing/end_to_end.rst

@@ -80,9 +80,9 @@ To register the Panther extension, add the following lines to ``phpunit.xml.dist
 
 .. code-block:: xml
 
-    <!-- phpunit.xml.dist -->
+    <!-- phpunit.dist.xml -->
     <extensions>
-        <extension class="Symfony\Component\Panther\ServerExtension"/>
+        <bootstrap class="Symfony\Component\Panther\ServerExtension"/>
     </extensions>
 
 Without the extension, the web server used by Panther to serve the application