Fix typo in CONTRIBUTING.md (unit-test -> unit-tests)

[KentaTsujii]

unit-test -> unit-tests

resolves #N/A

Problem

In the CONTRIBUTING.md file, there is a typo where unit-test should be unit-tests. This does not affect end-users, but it helps keep the documentation consistent.

Solution

Corrected the typo from unit-test to unit-tests for consistency.

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required/relevant for this PR.
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX.