Fix sample code of alterF in haddock

Found by a question in StackOverflow https://stackoverflow.com/questions/52828746/haskell-data-map-strict-alterf-parse-error-on-input/52829097

Author: Yuji Yamamoto

Data/IntMap/Internal.hs

@@ -963,14 +963,14 @@ alter f k Nil     = case f Nothing of
 -- @
 -- interactiveAlter :: Int -> IntMap String -> IO (IntMap String)
 -- interactiveAlter k m = alterF f k m where
---   f Nothing -> do
+--   f Nothing = do
 --      putStrLn $ show k ++
 --          " was not found in the map. Would you like to add it?"
 --      getUserResponse1 :: IO (Maybe String)
---   f (Just old) -> do
---      putStrLn "The key is currently bound to " ++ show old ++
+--   f (Just old) = do
+--      putStrLn $ "The key is currently bound to " ++ show old ++
 --          ". Would you like to change or delete it?"
---      getUserresponse2 :: IO (Maybe String)
+--      getUserResponse2 :: IO (Maybe String)
 -- @
 --
 -- 'alterF' is the most general operation for working with an individual

Data/IntMap/Strict.hs

@@ -607,14 +607,14 @@ alter f !k t =
 -- @
 -- interactiveAlter :: Int -> IntMap String -> IO (IntMap String)
 -- interactiveAlter k m = alterF f k m where
---   f Nothing -> do
+--   f Nothing = do
 --      putStrLn $ show k ++
 --          " was not found in the map. Would you like to add it?"
 --      getUserResponse1 :: IO (Maybe String)
---   f (Just old) -> do
---      putStrLn "The key is currently bound to " ++ show old ++
+--   f (Just old) = do
+--      putStrLn $ "The key is currently bound to " ++ show old ++
 --          ". Would you like to change or delete it?"
---      getUserresponse2 :: IO (Maybe String)
+--      getUserResponse2 :: IO (Maybe String)
 -- @
 --
 -- 'alterF' is the most general operation for working with an individual

Data/Map/Internal.hs

@@ -1167,14 +1167,14 @@ data AreWeStrict = Strict | Lazy
 -- @
 -- interactiveAlter :: Int -> Map Int String -> IO (Map Int String)
 -- interactiveAlter k m = alterF f k m where
---   f Nothing -> do
+--   f Nothing = do
 --      putStrLn $ show k ++
 --          " was not found in the map. Would you like to add it?"
 --      getUserResponse1 :: IO (Maybe String)
---   f (Just old) -> do
---      putStrLn "The key is currently bound to " ++ show old ++
+--   f (Just old) = do
+--      putStrLn $ "The key is currently bound to " ++ show old ++
 --          ". Would you like to change or delete it?"
---      getUserresponse2 :: IO (Maybe String)
+--      getUserResponse2 :: IO (Maybe String)
 -- @
 --
 -- 'alterF' is the most general operation for working with an individual

Data/Map/Strict/Internal.hs

@@ -814,14 +814,14 @@ alter = go
 -- @
 -- interactiveAlter :: Int -> Map Int String -> IO (Map Int String)
 -- interactiveAlter k m = alterF f k m where
---   f Nothing -> do
+--   f Nothing = do
 --      putStrLn $ show k ++
 --          " was not found in the map. Would you like to add it?"
 --      getUserResponse1 :: IO (Maybe String)
---   f (Just old) -> do
---      putStrLn "The key is currently bound to " ++ show old ++
+--   f (Just old) = do
+--      putStrLn $ "The key is currently bound to " ++ show old ++
 --          ". Would you like to change or delete it?"
---      getUserresponse2 :: IO (Maybe String)
+--      getUserResponse2 :: IO (Maybe String)
 -- @
 --
 -- 'alterF' is the most general operation for working with an individual