Update 404.html

[liliyao2022]

Replaced the nav tag with the header tag.

Fixes #0000

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[gitpod-io]

[liliyao2022]

replaced the nav tag with the header tag.

[codeclimate]

Code Climate has analyzed commit 8db22fd and detected 0 issues on this pull request.

View more on Code Climate.

[github-actions]

This pull request generated screenshots of many common pages in the running app. You should be able to download and view them here:
https://github.com/publiclab/plots2/suites/8800897261/artifacts/400190952

[TildaDares]

Suggested change

	<hearder id="header" class="navbar navbar-dark bg-dark fixed-top mb-5 py-3">
	<header id="header" class="navbar navbar-dark bg-dark fixed-top mb-5 py-3">

[TildaDares]

Suggested change

	</hearder>
	</header>

[TildaDares]

Hi @liliyao2022, can you add a link to the issue?

One thing that can help to get started is to make sure you've included a link back to the original issue you're solving, in the
format fixes #0000 (for example)

[TildaDares]

Thank you for taking the time to work on this PR but the issue has been resolved in #11539.