minor broken link in binary submodule

Dear libraries, I'm reporting about minor broken link in source comment. There is a broken link in `libraries/binary/src/Data/Binary/Get.hs`. --- <http://www.haskell.org/ghc/docs/latest/html/users_guide/ pragmas.html#unpack-pragma> +-- <http://www.haskell.org/ghc/docs/latest/html/users_guide/ glasgow_exts.html#unpack-pragma> Regards, Takenobu