spec: Update dir-spec.txt with HS v3 consensus param
We have a series of consensus param in the HS v3 subsystem that needs to be documented in the dir-spec.txt namely:
- hs_intro_min_introduce2
- hs_intro_max_introduce2
- hs_intro_min_lifetime
- hs_intro_max_lifetime
- hs_intro_num_extra
- hsdir_n_replicas
- hsdir_spread_fetch
- hsdir_spread_store
- hsdir-interval
Probably that last one we want to change it to hsdir_interval
(in the code as well) so we follow the proper syntax of using underscore.