Change the unknown filtered value to "@unknown" from "_"

Author: guohuideng2024

fetch.bs

@@ -6321,7 +6321,7 @@ optional boolean <var>forceNewConnection</var> (default false), run these steps:
          <li><p>Let <var>codings</var> be the result of <a>extracting header list values</a> given
          `<code>Content-Encoding</code>` and <var>response</var>'s <a for=response>header list</a>.
 
-         <li><p>Let <var>filteredCoding</var> be "<code>_</code>".
+         <li><p>Let <var>filteredCoding</var> be "<code>@unknown</code>".
 
          <li><p>If <var>codings</var> is null or failure, then set <var>filteredCoding</var> to
          the empty string.