Fix CONTRIBUTING.md instructions to install multiple Python versions (

astral-sh#5015)

## Summary

I noticed the command to install multiple Python versions was wrong as
it was failing cause `toolchain` is not a known command.

I looked in the `ci.yml` workflow to see which command is used there and
updated the instructions accordingly.

## Test Plan

I just ran the command locally. :)

CONTRIBUTING.md

@@ -39,7 +39,7 @@ If tests fail due to a mismatch in the JSON Schema, run: `cargo dev generate-jso
 Testing uv requires multiple specific Python versions; they can be installed with:
 
 ```shell
-cargo run toolchain install
+cargo run python install
 ```
 
 The storage directory can be configured with `UV_PYTHON_INSTALL_DIR`.