Change rule count from 500 to 600 (astral-sh#6605)

Fixes missing change from PR astral-sh#6579.

README.md

@@ -30,7 +30,7 @@ An extremely fast Python linter, written in Rust.
 - 🤝 Python 3.11 compatibility
 - 📦 Built-in caching, to avoid re-analyzing unchanged files
 - 🔧 Autofix support, for automatic error correction (e.g., automatically remove unused imports)
-- 📏 Over [500 built-in rules](https://beta.ruff.rs/docs/rules/)
+- 📏 Over [600 built-in rules](https://beta.ruff.rs/docs/rules/)
 - ⚖️ [Near-parity](https://beta.ruff.rs/docs/faq/#how-does-ruff-compare-to-flake8) with the
     built-in Flake8 rule set
 - 🔌 Native re-implementations of dozens of Flake8 plugins, like flake8-bugbear