[synthetics] Update tests with HMAC auth docs

[ruizb]

What does this PR do? What is the motivation?

Update a link in the guide to create Synthetics tests using HMAC authentication.

Merge instructions

Merge will be done once the work on engineering side is complete.

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/ruizb/synthetics/update-hmac-auth-guide/synthetics/guide/http-tests-with-hmac

[ruizb]

/merge

[dd-devflow[bot]]

:steam_locomotive: MergeQueue: pull request added to the queue

The median merge time in master is 25m.

Use /merge -c to cancel this operation!