docs: fix math rendering in box example

[sai-kalai]

The LaTeX rendering was broken in one line of the box example due to incorrect usage of matching \left. for every \right|

[sai-kalai]

Solves #3237 @wsmoses

[sai-kalai]

I am unsure if this will pass the CI pipeline, because when I try to build the docs myself by running

julia --project=docs docs/make.jl -- doctest=false  

from the project's root, I get an error. I'm unsure what I need to do to be able to build the docs.

 Error: failed to run `@example` block in docs/src/generated/box.md:586-588
│ ```@example box
│ @show abs.(diffs .- dstate_old[5])./dstate_old[5]
│ ```
│   exception =
│    UndefVarError: `diffs` not defined in `Main.__atexample__named__box`
│    Suggestion: check for spelling errors or missing imports.

[wsmoses]

we should fix that too [cc @swilliamson7 ], though we can get this merged first