chore: update script example from hybrid performance guide

[heitortsergent]

What?

The example in our Hybrid performance with k6 browser guide included a couple of variables that weren't defined, and the example won't work if a user is trying to copy/paste it.

I updated the example so that users can copy/paste the script and run it, and it should work.

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)

[heitortsergent]

@ankur22 @mdcruz would you mind taking a look at this and see if the changes I made to the example script make sense? 🤓

[heitortsergent]

I reviewed this locally and with @tom-miseur, so I'm going to go ahead and merge this. 🤓