Added example for running specific scenario via cli

[taylorflatt]

Why?

On a number of occasions I have found colleagues wanting to run a specific scenario of a given test script. Early on I saw folks would just comment things out to get to the behavior they wanted or would start nesting scripts and fanning tests out.

What?

• Added an example to the scenario section outlining how to effectively use an environment variable to run a specific scenario among a group.
• Tested example in all versions that it works as expected.

Checklist

• I have used a meaningful title for the PR.
• I have described the changes I've made in the "What?" section above.
• I have performed a self-review of my changes.
• I have run the npm start command locally and verified that the changes look good.

• I have made my changes in the docs/sources/next folder of the documentation.
• I have reflected my changes in the docs/sources/v{most_recent_release} folder of the documentation.
• I have reflected my changes in the relevant folders of the two previous k6 versions of the documentation (if still applicable to previous versions).

Related PR(s)/Issue(s)

• Option to toggle the scenario run k6#2780

[taylorflatt]

Any thoughts @heitortsergent?