doc: make Web Crypto example spec compliant

subtle.sign is not supposed to support strings, and in most Web Crypto
implementations, it does not. Passing a string as the 'data' argument
only works in Node.js, and users should not rely on that oddity. The
Web Crypto spec requires the data argument to be a BufferSource, i.e.,
an ArrayBuffer or an ArrayBufferView.

PR-URL: nodejs#41556
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/webcrypto.md

@@ -19,9 +19,12 @@ const { subtle } = require('crypto').webcrypto;
     length: 256
   }, true, ['sign', 'verify']);
 
+  const enc = new TextEncoder();
+  const message = enc.encode('I love cupcakes');
+
   const digest = await subtle.sign({
     name: 'HMAC'
-  }, key, 'I love cupcakes');
+  }, key, message);
 
 })();
 ```