fix: doctest ignore is used for math expressions that are not valid Rust

[YeungOnion]

The ignore option for doctests is supposed to ignore ``` segments from being treated as a doctest but will be run with all the tests that have the #[ignore] directive. What the rustdoc feature should do isn't up to us, but I do think that there's a better way to share this information that clearly conveys that this is not a doctest.

KaTeX is an option example, markdown styled quotes, or monospaced text using text instead of ignore

[YeungOnion]

opted for text in lieu of ignore in PR #213