Update usability-testing.md

[annepetersen]

removed extraneous word

Description of changes

Describe what this pull request is changing. Try to use plain language as much
as possible to help teammates understand what they are reviewing.

Known issues

If this pull request introduces anything that needs to be addressed later,
note it here. Some examples might be if you introduced a new link that fails
tests but will work when another pull request is merged; you created a new
method placeholder but it does not yet have content; or your change breaks
something else that will be removed later. If you create any issues to
address issues created by this pull request, this is a good place to list
them! You can delete this section if the PR does not introduce any new issues.

Related issues

If this pull request is associated with any other issues or pull requests,
lis them here. If this PR closes an issue, list it as closes #issue so
the issue will automatically be closed when the PR is merged. If there are no
related issues or PRs, you can delete this section.

Reviewers:

In addition to assigning reviewers in the GitHub interface, please also list
who you would like a review from here and, if applicable, what kind of review
you would like from them. For example, you might ask one person to review
content while someone else reviews visual layout.

• @username: please review content
• [ ]
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