Update setting-locale-without-routing-parameter.md

Fix the removing the locale segment so it works with both formats : `de` *and* `de_DE`

Author: jbelien

docs/book/cookbook/setting-locale-without-routing-parameter.md

@@ -60,7 +60,7 @@ class SetLocaleMiddleware implements MiddlewareInterface
         $this->helper->setBasePath($locale);
 
         return $delegate->process($request->withUri(
-            $uri->withPath(substr($path, 3))
+            $uri->withPath(substr($path, strlen($locale)+1))
         ));
     }
 }