Update content_security_policy.md

[davidbielik]

Why are you making this change? (required)

Adding warning to connect-src to ensure API endpoint url matches the chosen baseURL.

Related PRs, issues, or features (optional)

• N/A

Feature release date (optional)

• N/A

Contributor checklist

• I confirm that my PR meets the following:
  • I signed the Contribution License Agreement (CLA).
  • My style and voice follows the Braze Style Guide.
  • My content contains correct spelling and grammar.
  • All links are working correctly.
  • If I renamed or moved a file or directory, I set up URL redirects for each file.
  • If I updated or replaced an image, I did not remove the original image file from the repository. (For more information, see Updating an image).
  • If my PR is related to a paid SKU, third party, SMS, AI, or privacy, I have received written approval from Braze Legal.

Submitting for review

If your PR meets the above requirements, select Ready for review, then add a reviewer:

• Non-Braze contributors: Tag @braze-inc/docs-team in a comment below.
• Braze employees: Have any relevant subject matter experts (like engineers or product managers) review your work first, then add the tech writer assigned to your specific vertical. If you're not sure which tech writer to add, you can add braze-inc/docs-team instead.

Thanks for contributing! We look forward to reading your work.

[internetisaiah]

LGTM