Fix "Benchmark Type" code block in compensation-calculator

Why is this change being made?

Hey, I noticed that the code block in the "SF Benchmark > Benchmark Types" section on the compensation-calculator index page is not correctly formatted. It only shows as inline text and is hardly readable. To fix that, I added the three back-ticks to make it a Markdown code block.

Link to the page and section in the handbook: https://about.gitlab.com/handbook/total-rewards/compensation/compensation-calculator/#benchmark-types

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per [say-why-not-just-what][transparency]
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in [#mr-buddies][mr-buddies-slack].
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.