fixed legacy PHP syntax in Optional Routing

[Dmitrev]

The current docs contains some unrecommended legacy PHP syntax to make types nullable.

See PHP docs for referencece:
https://www.php.net/manual/en/language.types.declarations.php

Note:

It is also possible to achieve nullable arguments by making null the default value. This is not recommended as if the default value is changed in a child class a type compatibility violation will be raised as the null type will need to be added to the type declaration.

Example #2 Old way to make arguments nullable

<?php
class C {}

function f(C $c = null) {
   var_dump($c);
}

f(new C);
f(null);
?>