fix: typo in "why write tests" section

[Izzy-Snyder]

Fixes #3534

Changes:

• Fixed a single word typo in the "why write tests" section of the testing developer docs
• This was a learning project for the p5.js editor Open Source Internship - I was learning how to make a pull request!

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

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[raclim]

Thanks so much for your work on this, this is great! :)