Fix whitespace consistency in config.md

[tianon]

Some of the sub-items of config were inconsistently indented (some using three spaces, some using two) -- adjusted to two to match the rest of the file.

[tianon]

I was looking at why https://specs.opencontainers.org/image-spec/config/?v=v1.0.1#properties is rendering the sub-items of config as being even with it instead, and noticed this inconsistency. I'm not 100% sure which engine that's using to render (or how to most effectively test it), but IMO this seems like a sane change even if it's not the cause of that rendering glitch. 😅

[tianon]

The CI-generated PDF renders correctly, for what that's worth!

(It was probably fine with the janky whitespace too, though)

[sudo-bmitch]

LGTM. Children of rootfs and history should also be adjusted to 2 spaces for consistency. And the bold and italic markup also needs work. But those can all be a separate PR, especially since it needs to happen in other files, too. I've got on my list to add a markdown linter here after doing a cleanup.

[vbatts]

🧁