We seem to use `npm test`, not `gulp test`

[mattwynne]

Description

Change contributing docs to use npm test not gulp test

Motivation and Context

I wanted to contribute, and when I followed this step I got an error. Not even npx gulp test worked, and there's no gulpfile here, so I figured this step was out of date.

Checklist:

• [x] npm test passes without errors
• [x] I have read the contribution guidelines
• [x] I have updated the documentation accordingly

[eriwen]

Thanks for the contribution!