Remove unnecessary repetition in instrumenthttp.md

[tom-miseur]

Fix unnecessary repetition

What?

A simple removal of some unnecessary repetition. LMK if this also needs to go into another repo like /next

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/k6/next folder of the documentation.

• I have reflected my changes in the docs/sources/k6/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).

• I have made my changes in the docs/sources/k6/next folder of the documentation.

Related PR(s)/Issue(s)

n/a

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[heitortsergent]

@tom-miseur thanks for catching that! And yeah, if you could please port this to next as well, and then sign the CLA (it's reset, so it's asking all users to do it again for this repo), I can go ahead and merge this. :)