[llvm] [GlobalISel][Docs] Fix known-bits example and code snippet in KnownBits.rst (PR #213090)

[Kevin Le via llvm-commits]

kneevin wrote:

I've read the contribution policies, including the LLVM AI Tool Use Policy, the Code-Review Policy, and the Developer Policy. I used an AI assistant to sanity-check that the known-bits `AND` result is correct.

https://github.com/llvm/llvm-project/pull/213090