Clean up cross references/versioning

In https://github.com/whatwg/fetch/pull/1202 we're making Fetch's integration with Resource Timing more concrete. This creates a weird situation however where we "Mark resource timing" ends up pointing to https://www.w3.org/TR/resource-timing-2/#dfn-mark-resource-timing whereas otherwise we reference the editor's draft at https://w3c.github.io/resource-timing/. This also leads to a double entry in the References section with both RESOURCE-TIMING and RESOURCE-TIMING-2. (Disregard for now that one is marked as non-normative. That's a bug in Fetch though it's mostly because of this since if the term would point to the editor's draft as expected it would be associated with RESOURCE-TIMING and it would all be good.)