Fix typo and update ProjectEuler library

Haskell/ProjectEuler.hs

@@ -1,7 +1,10 @@
 module ProjectEuler
 ( isPrime
+, digitSum
 ) where
 
+import Data.Char (digitToInt)
+
 isPrime :: (Integral n) => n -> Bool
 isPrime 1 = False
 isPrime 2 = True
@@ -10,3 +13,8 @@ isPrime n =
     n `mod` 2 /= 0 && n `mod` 3 /= 0 && null [ x | x <- candidates, n `mod` x == 0 || n `mod` (x+2) == 0 ]
     where candidates = [5,11..limit]
           limit = floor(sqrt(fromIntegral n)) + 1
+
+
+digitSum :: (Integral a, Show a) => a -> Int
+digitSum n = sum $ map digitToInt $ show n
+

Haskell/p020.hs

@@ -13,5 +13,5 @@ factorial n = n * factorial (n - 1)
 
 main = do
     let result = digitSum $ factorial 100
-    putStrLn $ "Porject Euler, Problem 20\n"
+    putStrLn $ "Project Euler, Problem 20\n"
         ++ "Answer: " ++ (show result)