Add missing "s"

[wpgoble]

In the second bullet point under "Resources" in the "End-to_End Tests" section of the ONBOARDING.md file, the word "Cypress" is mispelled as "Cypres" and should be corrected. Also, the word "work" should be pluralized to "works"

[fejesievo]

I would like to work on this please!

[github-actions]

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