Fix Missing "t" Typo

[braughtg]

Change "ext" to "text" in the phrase " (e.g. button, ext field)" at the end of bullet point 3 under "End-To-End" tests in the "Cypress" section of ONBOARDING.md.

[brigidfoley]

I would like to work on this please!

[github-actions]

Great! I assigned you (@brigidfoley) to the issue. Have fun working on it!