fix some mixed space+tab indentation

This fixes all instances of " \t" in the code base. Most common case
seems to be after a "where" line; probably vim copied the two space layout
of that line.

Done as a background task while listening to episode 2 of the Type Theory
podcast.

Utility/HumanTime.hs

@@ -47,8 +47,8 @@ daysToDuration i = Duration $ i * dsecs
 parseDuration :: String -> Maybe Duration
 parseDuration = Duration <$$> go 0
   where
-  	go n [] = return n
-  	go n s = do
+	go n [] = return n
+	go n s = do
 		num <- readish s :: Maybe Integer
 		case dropWhile isDigit s of
 			(c:rest) -> do