Added example for X-Content-Type-Options in Fetch Standard

[SaaraBilyour]

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.

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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)
• 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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