[`pyflakes`] Fix backslash in docs (`F621`) (#19098)

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

This fixes the docs for [expressions-in-star-assignment
(F621)](https://docs.astral.sh/ruff/rules/expressions-in-star-assignment/#expressions-in-star-assignment-f621)
having a backslash `\` before the left shifts `<<`. I'm not sure why
this happened in the first place, as the docstring looks fine, but
putting the `<<` inside a code block fixes it. I was not able to track
down the source of the issue either. The only other rule with a `<<` is
[missing-whitespace-around-bitwise-or-shift-operator
(E227)](https://docs.astral.sh/ruff/rules/missing-whitespace-around-bitwise-or-shift-operator/#missing-whitespace-around-bitwise-or-shift-operator-e227),
which already has it in a code block.

Old docs page:

![image](https://github.com/user-attachments/assets/993106c6-5d83-4aed-836b-e252f5b64916)
> In Python 3, no more than 1 \\<< 8 assignments are allowed before a
starred expression, and no more than 1 \\<< 24 expressions are allowed
after a starred expression.

New docs page:

![image](https://github.com/user-attachments/assets/3b40b35f-f39e-49f1-8b2e-262dda4085b4)
> In Python 3, no more than `1 << 8` assignments are allowed before a
starred expression, and no more than `1 << 24` expressions are allowed
after a starred expression.

## Test Plan

<!-- How was it tested? -->

N/A, no tests/functionality affected.

crates/ruff_linter/src/rules/pyflakes/rules/starred_expressions.rs

@@ -11,8 +11,8 @@ use crate::{Violation, checkers::ast::Checker};
 /// ## Why is this bad?
 /// In assignment statements, starred expressions can be used to unpack iterables.
 ///
-/// In Python 3, no more than 1 << 8 assignments are allowed before a starred
-/// expression, and no more than 1 << 24 expressions are allowed after a starred
+/// In Python 3, no more than `1 << 8` assignments are allowed before a starred
+/// expression, and no more than `1 << 24` expressions are allowed after a starred
 /// expression.
 ///
 /// ## References