*: Use four-space indents for list-entry continuation

[wking]

The Markdown spec is liberal for the first paragraph:

To make lists look nice, you can wrap items with hanging indents… But if you want to be lazy, you don’t have to…

However, it's a bit more strict about subsequent paragraphs:

List items may consist of multiple paragraphs. Each subsequent paragraph in a list item must be indented by either 4 spaces or one tab…

That doesn't matter for our use here, because all of our entries are single-paragraph. But runtime-spec has been bitten by Pandoc strictness for multiple paragraphs before (opencontainers/runtime-spec#495), and their RELEASES.md has used four-space indents since (opencontainers/runtime-spec#846). By adopting the stricter behavior here, we make it easier for OCI Projects to stay synchronized with the template while maintaining their stricter local conventions. OCI Projects that do not have strict local conventions probably don't care either way.