Adopt src/.preload.php

Author: nicolas-grekas

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.