Use strict monad in sample (#553)

Author: gusty

src/FSharpPlus.Docs/Samples/Learn You a Haskell.fsx

@@ -563,7 +563,7 @@ let loadedCoin = Prob [(Heads, 0.1); (Tails, 0.9)]
 // Prob [(false, 0.025); (false, 0.225); (false, 0.025); (false, 0.225);
 //       (false, 0.025); (false, 0.225); (false, 0.025); (true, 0.225)]
 let flipThree : Prob<bool> =
-    monad {
+    monad' {
         let! a = coin
         let! b = coin
         let! c = loadedCoin