docs: update contributing guide (#4428)

CONTRIBUTING.md

@@ -106,7 +106,7 @@ At a high level, the steps involved in adding a new lint rule are as follows:
 1. Create a file for your rule (e.g., `crates/ruff/src/rules/flake8_bugbear/rules/abstract_base_class.rs`).
 1. In that file, define a violation struct. You can grep for `#[violation]` to see examples.
 1. Map the violation struct to a rule code in `crates/ruff/src/registry.rs` (e.g., `E402`).
-1. Define the logic for triggering the violation in `crates/ruff/src/checkers/ast.rs` (for AST-based
+1. Define the logic for triggering the violation in `crates/ruff/src/checkers/ast/mod.rs` (for AST-based
    checks), `crates/ruff/src/checkers/tokens.rs` (for token-based checks), `crates/ruff/src/checkers/lines.rs`
    (for text-based checks), or `crates/ruff/src/checkers/filesystem.rs` (for filesystem-based
    checks).