Test and then Remove the description in the README about rebuilding the image.

[ColinChamber]

As https://github.com/apache/incubator-kvrocks/pull/1415 has been merged, users no longer need to rebuild the image to use kvrocks-operator.

• [x] Try creating a cluster using the apache/kvrocks:nightly image.
• [x] If everything is OK, update the README.

[xiao-jay]

I can try to solve this issue