Update index.mdx about important remarks

[CFSeiya]

In Important remarks
changed
From:
You cannot modify or remove HTTP response headers whose name starts with cf- or x-cf-.

To:
You cannot add, modify or remove HTTP response headers whose name starts with cf- or x-cf-.

Summary

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[holopin-bot]

Congratulations @CFSeiya, the maintainer of this repository has issued you a holobyte! Here it is: https://holopin.io/holobyte/cmfxqcmw40007l4044byfj1y8

This badge can only be claimed by you, so make sure that your GitHub account is linked to your Holopin account. You can manage those preferences here: https://holopin.io/account.
Or if you're new to Holopin, you can simply sign up with GitHub, which will do the trick!