[`flake8-bandit`] Fix broken link (`S704`) (#22039)
Summary
--
While going through all the rules I noticed a broken link in the S704
docs. (I then got really confused because it appeared to be correct in
the _other_ `unsafe_markup_use.rs` file for the removed RUF version of
the rule).
Test Plan
--
[Before](https://docs.astral.sh/ruff/rules/unsafe-markup-use/):
<img width="537" height="171" alt="image"
src="https://github.com/user-attachments/assets/01007cab-6673-48e5-b3a5-6006bc78a027"
/>
After:
<img width="451" height="189" alt="image"
src="https://github.com/user-attachments/assets/4e5d0e0d-76be-4f66-b747-e209f11ab11a"
/>
I also did at least a cursory grep for other cases with escaped link
brackets (`\[`) and only turned up this rule on main.
