Updated the Fish Pond tutorial:

• added the missing app.stage.addChild(background); in the step 2 readme and moved the centering logic closer together
• added declared but unused deltaTime to the computation in step 3
• removed unused app parameter from animateWaterOverlay function in step 4
• fixed a bug that caused water animation to speed up every time the playground was updated (except for the updates in a file with the water animation itself). In order to achieve that, functions addFishes and addWaterOverlay now return a value that is passed further to the animation in Ticker.
• fixed a mismatch between redme code and the code in the playground in animateWaterOverlay in step 4
• fixed some typos