adjust markup and field names for packet dump

Apologies, it seems my PR didn't use appropriate markup for the packet dump. This PR (hopefully) fixes that.

It also proposes adjustments for the labels next to the hex dump, in an attempt to better align with naming in similar dumps shown in draft-ietf-openpgp-crypto-refresh. Let me know if that change seems good to you, or if you think something about it is suboptimal. I'm happy to adjust of course.