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
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documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc