Add missing @ in GHC.Float haddock

Documentation of Floating class is rendered as follows on Hackage, which contains extraneous at-mark in the exp (a + b) = @exp a * exp b line.

class Fractional a => Floating a where Source#

Trigonometric and hyperbolic functions and related functions.

The Haskell Report defines no laws for Floating. However, '(+)', '(*)' and exp are customarily expected to define an exponential field and have the following properties:

* exp (a + b) = @exp a * exp b
* exp (fromInteger 0) = fromInteger 1

This merge request fixes the problem.