Fix terminology mixup

[frivoal]

Fixes a minor terminology mixup in https://w3c.github.io/w3process/#change-review

It's clear from context and other related parts what is meant here, but the correct word to use in that sentence is "Changes", not "Additions".

For instance, this bit of related text is written as if we got it right from the first place.

The section says:

At the end of the Last Call for Review of Proposed Changes, the W3C
Decision may either be ...

and "W3C Decision" can only be done after AC Review:

A W3C decision is one where the Director decides, after exercising the
role of assessing consensus of the W3C Community after an Advisory
Committee review.

[fantasai]

Does this need to go into Process 2020?

[frivoal]

I think it could, but I don't think it needs to. This is an editorial improvement, so it accepting it should be safe and useful, but I'm not sure I want to disturb the P2020 approval cycle for non essential reasons.