minor #14176 Fix path to preload file (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
----------

Fix path to preload file

According to symfony/symfony#38063

Commits
-------

b830395 Fix path to preload file

Author: xabbuh

performance.rst

@@ -119,7 +119,9 @@ The preload file path is the same as the compiled service container but with the
 .. code-block:: ini
 
     ; php.ini
-    opcache.preload=/path/to/project/var/cache/prod/srcApp_KernelProdContainer.preload.php
+    opcache.preload=/path/to/project/src/preload.php
+
+This file is generated by the ``cache:clear`` command.
 
 .. _performance-configure-opcache: