minor #14208 Adopt src/.preload.php (nicolas-grekas)

This PR was merged into the 4.4 branch.

Discussion
----------

Adopt src/.preload.php

Follows symfony/symfony#38142

Commits
-------

08585c5 Adopt src/.preload.php

Author: javiereguiluz

performance.rst

@@ -119,7 +119,7 @@ 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/src/preload.php
+    opcache.preload=/path/to/project/src/.preload.php
 
 This file is generated by the ``cache:clear`` command.