[markdown mode] Add dash in `header-2`, `formatting-header-4` styles for consistency.

CM generally tends to use dashes before numbers, e.g. `variable-2`.
And markdown mode clearly went this way with `quote-3`, `formatting-quote-3` etc.
Seems better to fix while it's fresh rather than keep inconsistent forever. 
@bfrohs, @lukasoppermann any objections?