Update ch12-perf-testing

Good to merge 👍

Technical Correctness

[x] Are the filenames and paths in the docs consistent with the example code?
[ ] Are the config files in the docs consistent with the example code?
[ ] Could you follow along easily?
[x] Do all commands and configuration have an explanation of what they do?
[x] Do all commands and configuration have context of why they're necessary?
[x] Does the chapter include a way for the reader to verify that the infrastructure they set up works and accomplishes the goal of the chapter?

[x] Does the chapter follow the structure of the chapter template*?
[x] Does the chapter use consistent voice (e.g. first person plural) throughout?
[ ] Do you include links to within the GitHub repository when you mention files?

[x] Did a spellchecker catch any typos?
[x] Are all technical terms capitalized consistently and correctly?
[x] Are there any TODOs that can be moved to GitHub issues — or just addressed? (#52)
[ ] Are there any DO product docs links that need to be updated from Community to digitalocean.com/docs?
[ ] Do you use code formatting for commands and filenames? May need to review for style/design