[doc] Update demo code in readme doc

[Tiny-Box]

Related issue = #

[Taichi] version 0.8.11, llvm 10.0.0, commit 9d0973e, osx, python 3.9.5

When I tried to run taichi with the demo in the readme, I found that the demo code in the readme has not been updated for a long time, and this code does not work in the current version.
After prompting I found the code for fractal.py, which is updated in the readme.

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[k-ye]

Ouch, thanks for catching this. LGTM!