[DominanceFrontierBase] Fix doc of compare()'s return value. (#81352)

This is a trivial fix ( I guess it has not been noticed because of no
use). Currently, the doc says the function returns `true` if two
instances of `DominanceFrontierBase` matches, otherwise `false` is
returned. I have checked the implementation


https://github.com/llvm/llvm-project/blob/9308d6688c673606fee1625d777a52539ae72015/llvm/include/llvm/Analysis/DominanceFrontierImpl.h#L71-L94

which examines whether two dominance frontier mappings are equal, and
the actual value it returns contradicts the description in the doc.