Incorrect pretty-printing for warning categories

@barci2 helpfully pointed out some errors in the pretty-printing of warning categories that I introduced in !9684 (closed). For example, given this module:

module M {-# WARNING in "x-a" "b" #-} where

{-# WARNING in "x-c" e "d" #-}
e = e

the result of -ddump-parsed is:

module M
{-# WARNING "b" #-}
where
{-# WARNING [x-c] e "d" #-}
e = e

whereas it should be:

module M
{-# WARNING in "x-a" "b" #-}
where
{-# WARNING in "x-c" e "d" #-}
e = e