Fix haskell-docs invocation.

[gracjan]

Fixes #535.

[gracjan]

@lukehoersten: can you review and merge?

[lukehoersten]

Tested with:

test = toUpper <$> Just "hey"

and got the following failure (including *Messages* context):

Not in scope: ‘toUpper’
Identifier `toUpper' not in scope, choose module to import? (y or n) y
Invalid face reference: ido-first-match-face [106 times]
Haskell process command errored with: (wrong-type-argument listp t)

I don't think this patch fixes the issue.

[gracjan]

Ugh. But we are one step forward. What are these 'face' errors? Do you have some special ido setup?

[lukehoersten]

I only have default ido. ido-first-match-face isn't listed as a valid function or variable on my emacs.

Unfortunately I can't test if the <$> doc escape search problem is fixed because the other issue would cover it up.

[lukehoersten]

I tested with a clean emacs config and everything works fine. I tried and it works fine with ido in a clean config as well. Something else I have in my config is conflicting. We'll have to track it down but the <$> escape problem is fixed for sure.