Use/fix ron code blocks

[spenserblack]

• [ ] I've included my change in CHANGELOG.md (I don't think this counts as a "notable change")

Just a heads-up that GitHub now supports RON. This PR

• changes the rust,ignore code blocks to ron
• changes the ron (leading space) code blocks to ron

The downside of this PR is that, while GitHub now detects RON, other tools that render the markdown might not (like crates.io). So perhaps for that reason the README.md should be left alone and only the leading spaces should be fixed.

[juntyr]

Thank you @spenserblack for this PR and for pushing for Ron to be properly rendered on GitHub!