Added getPrefix function to be implemented.

cache.md

@@ -244,6 +244,7 @@ To create our custom cache driver, we first need to implement the `Illuminate\Co
   public function forever($key, $value) {}
   public function forget($key) {}
   public function flush() {}
+  public function getPrefix(){};
  }
 
 We just need to implement each of these methods using a MongoDB connection. Once our implementation is complete, we can finish our custom driver registration: