chore: Add missing ds-grey-400 on documentation

[vincentaudoire]

What this PR does

Adding missing grey-400 value on http://dirtyswan.design/?path=/story/css-variables-colors--page

Checklist:

• I reviewed my own code
• The changes align with the designs I received
  Or give a reason why this does not apply:
• I have attached screenshot(s), video(s) or gif(s) showing that the solution is working as expected
  Or give a reason why this does not apply:
• I have updated the task(s) status on Linear
• All new media is optimized (images, gifs, videos)

Browser support

My code works in the following browsers:

• Firefox
• Chrome
• Safari
• Edge