style fixes for markdown docs

[brendanjryan]

Adds some minor fixes to docs/index.md.

This should fix some broken formatting on https://kubeval.instrumenta.dev

@garethr -- what do you use to generate the docs for this app? I would like to verify that my changes work as expected

[garethr]

Thanks. The documentation uses MkDocs. You can find a mkdocs.yml and Pipfile in the root of the repo. I should add that to the Makefile and BUILDING.md` as well.

[brendanjryan]

ahh thank you! I was able to verify things 👍