Fix path to preload file

Author: nicolas-grekas

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: