Add heuristic to maze example

[PaulJackson123]

This is a very minor suggestion to add a heuristic to the maze example. It uses A* algorithm but without a heuristic it is equivalent to Dijkstra's algorithm.

I'm doing this as a learning exercise. I plan to submit a pull request. I'm new to Git, so apologies if I don't adhere to a best practice.

[gonzalezsieira]

This is fixed in 5825945. Thanks for your contribution!