doc: use serial comma in webstreams docs

Refs: nodejs/node#11321
Refs: nodejs/node#17384

PR-URL: nodejs/node#43353
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: tniessen

doc/api/webstreams.md

@@ -1252,7 +1252,7 @@ added: v16.6.0
   * `ignoreBOM` {boolean} When `true`, the `TextDecoderStream` will include the
     byte order mark in the decoded result. When `false`, the byte order mark
     will be removed from the output. This option is only used when `encoding` is
-    `'utf-8'`, `'utf-16be'` or `'utf-16le'`. **Default:** `false`.
+    `'utf-8'`, `'utf-16be'`, or `'utf-16le'`. **Default:** `false`.
 
 Creates a new `TextDecoderStream` instance.