[`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:
> 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:
> 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.
