Update PULL_REQUEST_TEMPLATE.md

[manez]

Purpose / why

Depends on #1683 being merged first.

Adds another checkbox to look for an updated "last updated on." Technically, this is included under the second checkbox, "Does the proposed documentation align with the Islandora Documentation Style Guide?", but I think that since the date should be common to all docs pages and it's probably going to be something that's easy to overlook, it merits its own callout in the checklist.

What changes were made?

Added a new checkbox for reviewers.

Verification

I think preview and review should do it for this one.

Interested Parties

@Islandora/8-x-committers once the other PR is merged.

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Checklist

Pull-request reviewer should ensure the following

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person merging should ensure the following

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?

[manez]

The Style Guide PR is in, so this is ready for review.