Fix npm command hint

Author: tshemsedinov

CONTRIBUTING.md

@@ -36,6 +36,6 @@ Before open pull request please follow checklist:
 
 - [ ] tests and linter show no problems (`npm t`)
 - [ ] tests are added/updated for bug fixes and new features
-- [ ] code is properly formatted (`npm run fmt`)
+- [ ] code is properly formatted (`npm run fix`)
 - [ ] description of changes is added in CHANGELOG.md
 - [ ] update .d.ts typings