Change serde_core's html_root_url to docs.rs/serde_core

[dtolnay]

Fixes #2977.

Maybe it would be possible to redirect trait definitions from docs.rs/serde_core to docs.rs/serde as the canonical location using something like this in --html-in-header:

<script>
  url_regex = /^https:\/\/docs\.rs\/serde_core\/([^\/]+)\/serde_core\/(.+)$/;
  serde_redirect = window.location.href.replace(url_regex, "https://docs.rs/serde/$1/serde/$2");
  if (serde_redirect !== window.location.href) {
    window.location.replace(serde_redirect);
  }
</script>