doc: fix indentation issue

If releases are indented, then that will cause release-post.js to render an odd looking blog post. Fixes the v8.8.1 changelog, and adds note to doc/releases.md.

Who knew two spaces could cause such trouble :)

Related PR: https://github.com/nodejs/nodejs.org/pull/1425

cc @cjihrig

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc