Align hash property format for HS512 mapping (#418)

In the JWK Algorithm Mappings table, the 'hash' property for the
HS512 algorithm is formatted as a string ("SHA-512").

This commit changes the format to be an object ({ name: 'SHA-512' }),
making it consistent with the format used by all other HMAC
algorithms (HS1, HS256, HS384) in the table.

Author: cardoso

spec/Overview.html

@@ -16310,7 +16310,8 @@ <h3>Algorithm mappings</h3>
                 <td>
 <pre class=js>
 { name: "HMAC",
-  hash: "SHA-512" }
+  hash: { name: "SHA-512" }
+}
 </pre>
                 </td>
               </tr>