Method getCharset with proper returning type

The method `getCharset` implemented by the example should return `string` type, like the `Symfony\Component\HttpKernel\Kernel` abstract class requires.

Author: githubfromgui

reference/configuration/kernel.rst

@@ -33,7 +33,7 @@ charset::
 
     class Kernel extends BaseKernel
     {
-        public function getCharset()
+        public function getCharset(): string
         {
             return 'ISO-8859-1';
         }