Update example to match Laravel 11's defaults

In Laravel 11, the base controller (i.e. App\Http\Controllers\Controller) no longer uses the ValidatesRequests trait, which means that $this->validate() is no longer available in the context of controllers by default.

readme.md

@@ -139,7 +139,7 @@ class SurveyEntriesController extends Controller
 {
     public function store(Survey $survey, Request $request)
     {
-        $answers = $this->validate($request, $survey->rules);
+        $answers = $request->validate($request, $survey->rules);
 
         (new Entry)->for($survey)->fromArray($answers)->push();
     }