doc: use length argument in pbkdf2Key

The optional `length` argument is supposed to determine the
length of the AES-GCM key, so it may be 128, 192, or 256.

PR-URL: #51066
Reviewed-By: Marco Ippolito <marcoippolito54@gmail.com>
Reviewed-By: Filip Skokan <panva.ip@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/webcrypto.md

@@ -330,7 +330,7 @@ async function pbkdf2Key(pass, salt, iterations = 1000, length = 256) {
     iterations,
   }, keyMaterial, {
     name: 'AES-GCM',
-    length: 256,
+    length,
   }, true, ['encrypt', 'decrypt']);
   return key;
 }