✨ Correct default value of DAST_SPIDER_START_AT_HOST in the docs

What does this MR do and why?

During gitlab-com/support/support-pairing#11107, we discussed the DAST_SPIDER_START_AT_HOST variable and we observed that the docs note that the default is for this variable to be true. However, the gitlab-org/security-products/dast!497 (merged) MR made it so that the default value for this variable is false.

This discrepancy was documented in #428792 (closed) and is fixed in this MR.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.