chore(docs): fix typo in testing contributor docs

[adi-ray]

Fixes #3534

Changes:Fixed the typo in testing.md

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

[welcome]

🎉 Thanks for opening this pull request! Please check out our contributing guidelines if you haven't already.

[raclim]

Thanks so much for opening this PR! Since the original issue has already been assigned to another contributor, we will move forward with the assigned contributor's contribution as we have try to abide by this guideline listed in the pinned issue "Notes to Keep in Mind":

"Please do not open a pull request for an issue that is already assigned to someone else. We follow a “first assigned, first served” approach to avoid duplicated work. If you open a PR for an issue that someone else is already working on, your PR will be closed."

Looking through our contributor docs, I'm realizing that this guideline is noted only within the pinned issue and not in the "Preparing a Pull Request" doc. I'm sorry that we couldn't get this in and for this discrepancy, but if you'd like, please feel free to open an issue and pull request to amend this section if you'd like to give a different issue a try!