Touch up formatting for variance README.md

[cbreeden]

There were a few places that needed extra indentation to register as a code block.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pnkfelix (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[cbreeden]

Nit: It's somewhat unfortunate that travis will spend nearly 2 hours running tests for something like this.

[GuillaumeGomez]

@bors: r+ rollup

[bors]

📌 Commit 42a5a06 has been approved by GuillaumeGomez