Remove duplicate line from project structure docs (#6408)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

CONTRIBUTING.md

@@ -131,7 +131,6 @@ At time of writing, the repository includes the following crates:
 - `crates/ruff_macros`: proc macro crate containing macros used by Ruff.
 - `crates/ruff_python_ast`: library crate containing Python-specific AST types and utilities.
 - `crates/ruff_python_codegen`: library crate containing utilities for generating Python source code.
-- `crates/ruff_python_codegen`: library crate containing utilities for generating Python source code.
 - `crates/ruff_python_formatter`: library crate implementing the Python formatter. Emits an
     intermediate representation for each node, which `ruff_formatter` prints based on the configured
     line length.