Fix typo in haskell example

To make literal behaving like regular function sorround operator with parens. To get opposite sorround function with backticks.

Author: olorin37

doc/Language/haskell-to-p6.pod6

@@ -438,7 +438,7 @@ does not translate exactly the same, but it's possible to do the same things, no
 Fold in Haskell is called Reduce in Raku.
 
     =begin code :lang<haskell>
-    mySum = foldl `+` 0 numList
+    mySum = foldl (+) 0 numList
     =end code
 
     =begin code