Update PEP reference in future_rewritable_type_annotation.rs (#11985)

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## Summary

Documentation mentions:

> PEP 563 enabled the use of a number of convenient type annotations,
such as `list[str]` instead of `List[str]`

but it meant [PEP 585](https://peps.python.org/pep-0585/) instead.

[PEP 563](https://peps.python.org/pep-0563/) is the one defining `from
__future__ import annotations`.

## Test Plan

No automated test required, just verify that
https://peps.python.org/pep-0585/ is the correct reference.

crates/ruff_linter/src/rules/flake8_future_annotations/rules/future_rewritable_type_annotation.rs

@@ -12,7 +12,7 @@ use crate::checkers::ast::Checker;
 /// PEP 563.
 ///
 /// ## Why is this bad?
-/// PEP 563 enabled the use of a number of convenient type annotations, such as
+/// PEP 585 enabled the use of a number of convenient type annotations, such as
 /// `list[str]` instead of `List[str]`. However, these annotations are only
 /// available on Python 3.9 and higher, _unless_ the `from __future__ import annotations`
 /// import is present.