add missing function call in recursion

src/05-first-logic.md

@@ -38,5 +38,5 @@ Recursion is main method of repetition (loops are not very functional)
 ```haskell
 doubleAll :: [Int] -> [Int]
 doubleAll [] = []
-doubleAll (x:xs) = (x * 2) : xs
+doubleAll (x:xs) = (x * 2) : doubleAll xs
 ```