Update dir-spec descriptor cosmetic difference criteria (12 hours -> 2 hours)

In section 3.2 of the dir-spec document it says:

If the authority _does_ have a descriptor with the same public key, the
newly uploaded descriptor is remembered if its publication time is more
recent than the most recent old descriptor for that router, and either:

   - There are non-cosmetic differences between the old descriptor and the
     new one.
   - Enough time has passed between the descriptors' publication times.
     (Currently, 12 hours.)

This value of 12 hours was change in commit cc351578 to 2 hours.

In addition, a comment in the tor code still says 12 hours.

This ticket covers two changes:

• update the dir-spec to say 2 hours
• update a comment in the tor code

Will attach two PRs shortly.