chapter3/parsec.hs: change definition of option

Let (<|>) (which equals option) return all parses of the succeeding parser,
as seems to be implied by the description in the chapter on Parsing.

Example:  parse (empty <|> ((unit 'a') `combine` (unit 'b'))) ""

chapter3/parsec.hs

@@ -54,9 +54,9 @@ failure = Parser (\cs -> [])
 
 option :: Parser a -> Parser a -> Parser a
 option  p q = Parser $ \s ->
-  case parse (mplus p q) s of
-    []     -> []
-    (x:xs) -> [x]
+  case parse p s of
+    []     -> parse q s
+    res    -> res
 
 satisfy :: (Char -> Bool) -> Parser Char
 satisfy p = item `bind` \c ->