docs: fix Headers.append() call to use two arguments

[DucMinhNe]

Problem

The "Merging with parent headers → Appending" example in docs/how-to/headers.md calls Headers.append() with a single combined string:

parentHeaders.append(
  "Permissions-Policy: geolocation=()",
);

But the Web Fetch Headers.append(name, value) API takes two arguments. Copy-pasting this throws at runtime:

TypeError: Headers.append: 2 arguments required, but only 1 found.

(reproduced on Node v22). The .set("Cache-Control", "...") example just below it in the same section already uses the correct two-argument form.

Fix

Split into the two-argument call — docs-only, one example:

parentHeaders.append(
  "Permissions-Policy",
  "geolocation=()",
);

[remix-cla-bot]

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- The Remix team

[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳

[brophdawg11]

Thanks!