Fixed nextest install line in CONTRIBUTING.md (astral-sh#9929)

## Summary

I noticed the example line in CONTRIBUTING.md:

```shell
cargo install nextest
```

Didn't appear to install the intended package cargo-nextest.


![nextest](https://github.com/astral-sh/ruff/assets/12672027/7bbdd9c3-c35a-464a-b586-3e9f777f8373)

So I checked what it [should
be](https://nexte.st/book/installing-from-source.html) and replaced the
line:

```shell
cargo install cargo-nextest --locked
```

## Test Plan

Just checked the cargo install appeared to give sane looking results

---------

Co-authored-by: Charlie Marsh <[email protected]>

CONTRIBUTING.md

@@ -26,6 +26,10 @@ Welcome! We're happy to have you here. Thank you in advance for your contributio
 - [`cargo dev`](#cargo-dev)
 - [Subsystems](#subsystems)
     - [Compilation Pipeline](#compilation-pipeline)
+    - [Import Categorization](#import-categorization)
+        - [Project root](#project-root)
+        - [Package root](#package-root)
+        - [Import categorization](#import-categorization-1)
 
 ## The Basics
 
@@ -80,7 +84,7 @@ We recommend [nextest](https://nexte.st/) to run Ruff's test suite (via `cargo n
 though it's not strictly necessary:
 
 ```shell
-cargo install nextest
+cargo install cargo-nextest --locked
 ```
 
 Throughout this guide, any usages of `cargo test` can be replaced with `cargo nextest run`,