I am of the opinion that we should make these legitimate tag synonyms, instead of just manually editing them, so that in the future the correct one shows up in new users' search suggestions.
In this case, I'm inclined to agree, as both seem like legit terms. What does everyone else think?
I do notice that one of the (2) items under age-of-exploration technically doesn't fall under the tag description that age-of-discovery currently has, since it is about China. Still, I'd be OK with wedging it in there.