doc: At long last fix ghc.mk
doc: At long last fix ghc.mk

The variable reference was incorrectly escaped, meaning that Sphinx
documentation was never installed.

haskell/haddock,Fuuzetsu/haddock,Fuuzetsu/haddock,Fuuzetsu/haddock,Fuuzetsu/haddock,haskell/haddock,haskell/haddock,haskell/haddock,haskell/haddock