minor #14303 Update location of preloading file (nicolas-grekas)

This PR was submitted for the master branch but it was merged into the 4.4 branch instead.

Discussion
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Update location of preloading file

Follows symfony/symfony#38343 and  symfony/recipes#825

Commits
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8213d9c Update location of preloading file

Author: javiereguiluz

performance.rst

@@ -119,7 +119,7 @@ can configure PHP to use this preload file:
 .. code-block:: ini
 
     ; php.ini
-    opcache.preload=/path/to/project/src/.preload.php
+    opcache.preload=/path/to/project/config/preload.php
 
 .. _performance-configure-opcache: