minor symfony#8393 Update autoload file (tabbi89)

This PR was merged into the 3.3 branch.

Discussion
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Update autoload file

Version 3.3 and 3.4 have now vendor/autoload.php as a default bootstrap file

Commits
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0cc2812 Update autoload file

Author: xabbuh

testing.rst

@@ -91,7 +91,7 @@ of your application::
     directory.
 
 Just like in your real application - autoloading is automatically enabled
-via the ``app/autoload.php`` file (as configured by default in the
+via the ``vendor/autoload.php`` file (as configured by default in the
 ``phpunit.xml.dist`` file).
 
 Running tests for a given file or directory is also very easy: