customNotFoundResponse in Guide/recipes.markdown gets a better typean…

…notation - known to work for versions from ihp 0.11 up to 0.15

Guide/recipes.markdown

@@ -407,7 +407,7 @@ with a 404:
 ```haskell
 import qualified Data.ByteString.Lazy as LBS
 
-customNotFoundResponse :: IO ()
+customNotFoundResponse :: (?context :: ControllerContext) => IO () 
 customNotFoundResponse = do
 page <- LBS.readFile "static/404.html"
 respondAndExit $ responseLBS status404 [(hContentType, "text/html")] page