Cabal QA message typo

[Johan Tibell]

I can't easily send patches from work so I'll report this bug before I
forget about it. There's a typo in
Distribution/PackageDescription/QA.hs

    ghcWarn "-O2" $
        "-O2 is rarely needed as it often prolong the compile time "
        ++ "with usually with little benefit."

Notice the extra "with."

-- Johan