Fixed nextest install line in CONTRIBUTING.md

[owenlamont]

Summary

I noticed the example line in CONTRIBUTING.md:

cargo install nextest

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

So I checked what it should be and replaced the line:

cargo install cargo-nextest --locked

Test Plan

Just checked the cargo install appeared to give sane looking results

[charliermarsh]

Thanks!