Remove need to readjust margins in `404.html`

Since some of the users prefer to just modify the placeholder 404
page instead of replacing it with their own, this commit makes it
easier to do that by allowing them to modify the content without
worrying about readjusting the margins.

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Note: The different in size compared to the old 404 is negligible:

 * old 404:

   original size:         1242 B
   gzipped size:           574 B

 * new 404:

   original size:         1274 B
   gzipped size:           570 B

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Ref: h5bp/html5-boilerplate#1567.
Close: h5bp/html5-boilerplate#1567.