Fix ComboSession implementation

The ComboSession example code is broken: since Cache::gc() returns void, parent::gc() is never called and garbage collection is never executed.

en/development/sessions.rst

@@ -280,7 +280,8 @@ something like::
         // Removes expired sessions.
         public function gc($expires = null)
         {
-            return Cache::gc($this->cacheKey) && parent::gc($expires);
+            Cache::gc($this->cacheKey);
+            return parent::gc($expires);
         }
     }