[whatwg/fetch] Added example for X-Content-Type-Options in Fetch Standard (PR #1809)

### Summary
This PR adds an example demonstrating how to use the `X-Content-Type-Options` HTTP header with the Fetch API.

### Why This Change?
- The `X-Content-Type-Options` header prevents MIME-type sniffing, improving web security.
- The Fetch Standard currently lacks a practical example for this header.
- This addition helps web developers skimming the spec to understand how to use it in real-world scenarios.

### Changes Made
- Added a `<pre class="example">` block with a Fetch API usage example.
- Assigned a unique `id="example-x-content-type-options"` for Bikeshed validation.
- Placed the example before the ABNF definition, following standard spec structure.

---

### **Checklist (N/A for This PR)**
Since this is an **editorial change**, some checklist items are **not applicable**:
- [ ] At least two implementers are interested (**N/A: Editorial Example**)
- [ ] Tests are written (**N/A: No functionality changes**)
- [ ] Implementation bugs are filed (**N/A: No browser changes**)
- [ ] MDN issue is filed (**Can be considered after PR merge**)
- [x] The commit message is clear

This PR is purely editorial, and no implementation changes are needed. Happy to adjust if maintainers have feedback! 🚀


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<a href="https://whatpr.org/whatwg-fetch/1809.html" title="Last updated on Feb 12, 2025, 5:34 AM UTC (67a4987)">Preview</a> | <a href="https://whatpr.org/fetch/1809/07662d3...67a4987.html" title="Last updated on Feb 12, 2025, 5:34 AM UTC (67a4987)">Diff</a>
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/1809

-- Commit Summary --

  * Fixed Bikeshed errors: added example ID and resolved duplicate header ID

-- File Changes --

    M fetch.bs (12)

-- Patch Links --

https://github.com/whatwg/fetch/pull/1809.patch
https://github.com/whatwg/fetch/pull/1809.diff

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