Fix forkAff example. (#108)

Author: alpacaaa

README.md

@@ -189,7 +189,7 @@ The `Control.Monad.Aff.AVar` module contains asynchronous variables, which are v
 
 ```purescript
 do v <- makeVar
-   forkAff do
+   _ <- forkAff do
      delay (Milliseconds 50.0)
      putVar v 1.0
    a <- takeVar v