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.