Fix syntax error on front-page of docs

I was trying out https://hackage.haskell.org/package/cron-0.2.1/docs/System-Cron.html, installed the library, pasted in the code and got:

```haskell
import Control.Concurrent
import Control.Monad
import Data.Time.Clock
import System.Cron

main :: IO ()
main = do
  forever do
    now <- getCurrentTime
    when (scheduleMatches schedule now) doWork
    putStrLn "sleeping"
    threadDelay 100000
  where doWork   = putStrLn "Time to work"
        schedule = hourly

-- Syntax error:
-- Main.hs:8:11: parse error on input `do'
-- [nix-shell:~/programming/haskell/cron-test]$ ghc --version
-- The Glorious Glasgow Haskell Compilation System, version 7.6.3
```

This fixes the simple syntax error.