Update committers.md

[manez]

Purpose / why

Add the latest i8 committer.

What changes were made?

Added @adam-vessey to the official list

Verification

Review that info is correct 😄

Interested Parties

@adam-vessey tradition says you get to merge this as your first act as an i8 committer, if are you so inclined.

---

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?
• Does this PR update the last updated on date on the documentation page?

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?