[11.x] Removing unused var assignment in Illuminate Router (#53575)

* [11.x] Removing unused var assignment in Illuminate Router

* fix: readability concerns

Author: Carnicero90

src/Illuminate/Routing/Router.php

@@ -709,7 +709,7 @@ protected function mergeGroupAttributesIntoRoute($route)
     {
         $route->setAction($this->mergeWithLastGroup(
             $route->getAction(),
-            $prependExistingPrefix = false
+            prependExistingPrefix: false
         ));
     }