fixed legacy PHP syntax (#9024)

Author: Dmitrev

routing.md

@@ -197,11 +197,11 @@ If your route has dependencies that you would like the Laravel service container
 
 Occasionally you may need to specify a route parameter that may not always be present in the URI. You may do so by placing a `?` mark after the parameter name. Make sure to give the route's corresponding variable a default value:
 
-    Route::get('/user/{name?}', function (string $name = null) {
+    Route::get('/user/{name?}', function (?string $name = null) {
         return $name;
     });
 
-    Route::get('/user/{name?}', function (string $name = 'John') {
+    Route::get('/user/{name?}', function (?string $name = 'John') {
         return $name;
     });