Data/Conduit/Combinators.hs: fix bad haddock markup

haddock fails as:
  Preprocessing library conduit-combinators-0.2.8.1...
  Haddock coverage:
   100% (  5 /  5) in 'Data.Conduit.Combinators.Internal'
  dist/build/tmp-19228/Data/Conduit/Combinators.hs:1803:5:
      parse error on input ‘!’

Fuuzetsu helped me to find what's exactly wrong:

(19:06:57) Fuuzetsu: slyfox^w_: I think it's bad markup although the error is not great
(19:07:11) Fuuzetsu: you can't documment unnamed constructor fields

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>