Fix HMAC JWK import key "use" check (#413)

Check that the JWK "use" field is "sig" rather than "sign".

Author: panva

spec/Overview.html

@@ -14739,7 +14739,7 @@ <h5>Import Key</h5>
                       <li>
                         <p>
                           If |usages| is non-empty and the {{JsonWebKey/use}} field of |jwk| is present and is
-                          not  "`sign`",
+                          not  "`sig`",
                           then [= exception/throw =] a
                           {{DataError}}.
                         </p>