docs(validator): Recommend npx to run validator

[TWiStErRob]

Changes

Using npx has all the same benefits as --global (e.g. caching), except it does not pollute the global namespace and installed dependencies.

This also works better on CI, shared environments, and for aliasing, because it's a simple one-liner.

Documentation (please check one with an [x])

• [x] I have updated the documentation, or
• [ ] No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• [x] Code inspection only, or
• [ ] Newly added/modified unit tests, or
• [ ] No unit tests but ran on a real repository, or
• [ ] Both unit tests + ran on a real repository