Fixed nextest install line in CONTRIBUTING.md (#9929)
## Summary
I noticed the example line in CONTRIBUTING.md:
```shell
cargo install nextest
```
Didn't appear to install the intended package cargo-nextest.
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 <charlie.r.marsh@gmail.com>
