Update broken link in k6 guide

[gmckeever]

Replacing broken link with new doc URL.

What does this PR do?

The current link takes us to a 404 page. This is a simple update to provide the correct URL.

Note: This may need to be updated for other locales.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If this PR includes a log pipeline, please add a description describing the remappers and processors.

[alai97]

👋 Hey @gmckeever, thanks for opening this fix! FYI we don't need to update non-English languages of our docs (https://datadoghq.atlassian.net/wiki/spaces/docs4docs/pages/3157920030/Frequently+asked+questions#Do-I-have-to-update-the-non-English-docs-when-I-make-a-change%3F) because the change gets trickled down through our translation vendor service, Transifex. I'll close https://datadoghq.atlassian.net/browse/DOCS-8942 :)