doc: address nits for src/README.md

Refs: https://github.com/nodejs/node/pull/30552#pullrequestreview-321954717

PR-URL: https://github.com/nodejs/node/pull/30693


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>