fix env variable

Author: theofidry

components/runtime.rst

@@ -509,7 +509,7 @@ Create Your Own Runtime Template
 
 In some cases the default generic runtime template generated (`vendor/autoload_runtime.php`)
 file is not enough. For example: you want to use a different entry point than
-``$_SERVER['FILE_SCRIPTNAME']`` or a different SAPI than the traditional ``cli``,
+``$_SERVER['SCRIPT_FILENAME']`` or a different SAPI than the traditional ``cli``,
 ``phpdbg`` or ``embed``.
 
 To use your own template, you can register it as follows: