Refactor 404 error page to use div instead of p for search form

[AndrewDongminYoo]

The 404 error page’s search form was wrapped in a <p> tag, but <p> cannot contain block-level elements like <form>. This change refactors the markup to use a <div> instead, which:

• Improves semantic correctness of the HTML
• Prevents any rendering glitches caused by invalid nesting

Files changed:

• doc/api/__404error.html (replace <p>…</p> around the form with <div>…</div>)

No behavior changes beyond markup.

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