[Bug][example][Jacobi solver] stable fluid

[Eydcao]

Describe the bug
Credits to @YuCrazing @yuanming-hu for finding the bug in stable_fluid.py example. Artifacts in stable_fluid.py example.

To Reproduce
Press the left mouse button and drag it to the left-down corner of the window, you will see a strange acceleration of the vortex and extremely white dye.

One bug about the wrong symbol of adding the correction term (by shi in Games 201 WeChat group#2, TODO credit his GitHub id here)

I also found that the bug exists in Jacobi_dual pressure passion solver if turned off, the bug goes away.

• Side note: I would love to fix this one if not urgent, maybe 1~2 days will suffice.

[Eydcao]

Hi I am going to close the issue since changing the symbol for correction term in bfecc and commenting the jacobi-dual is very simple.

I could help doing so after #1731 which affects this example file to avoid conflict. Or this can be done alongside @archibate working progress in that PR. Thanks for the patience and the rule of simplicity, which :> Saves a lot of bother