fix typo in comment

spotted by Sgeo: Typo in lens docs
[11:18am] Sgeo:  -- baz :: Lens (Foo a) (Foo b) a b
[11:18am] Sgeo:  quux :: Functor f => (a -> f b) -> Foo a -> f (Foo b)
[11:18am] Sgeo: http://ekmett.github.io/lens/frames.html

in #haskell
Merge pull request #396 from ggreif/patch-2

fix typo in comment