base: Fix documentation for forkIOWithUnmask
base: Fix documentation for forkIOWithUnmask

forkIOUnmasked has been deprecated for several years now. Update reference to
it. See #4858 and #5546.

ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc