Correct docs to say UBSan `objc-cast` is not part of `undefined` group

[seanm]

clang/docs/UndefinedBehaviorSanitizer.rst says:

"-fsanitize=undefined: All of the checks listed above other than float-divide-by-zero, unsigned-integer-overflow, implicit-conversion, local-bounds and the nullability-* group of checks."

I'm pretty sure objc-cast should be in that list and was just forgotten in https://reviews.llvm.org/D71491.

[seanm]

@vedantk this may be up your alley...

[JohnTitor]

Submitted a fix: https://reviews.llvm.org/D127407

[seanm]

lgtm. thanks.