batches: correctly pluralise when asking to close changesets

Created by: LawnGnome

This actually required a bit more rewording than just a single pluralize() call. I've updated the storybook to allow the total number of changesets to be changed; opening https://5f0f381c0e50750022dc6bf7-jawrqfbzzn.chromatic.com/?path=/story/web-batches-close-batchchangeclosealert--has-open-changesets and changing the totalCount knob is probably the best way to test this.