Fix typo in Manual.md
Fix typo in Manual.md

Three times, the manual says to run `runhaskell _build/run` to run the build system. That's the final executable to be built in the example.  `runhaskell Build.hs` is the Haskell file with the build system in it.
ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake,ndmitchell/shake