Editorial: align with IDL (void → undefined)

Author: autokagami

fetch.bs

@@ -5426,11 +5426,11 @@ typedef (sequence<sequence<ByteString>> or record<ByteString, ByteStrin
 interface Headers {
   constructor(optional HeadersInit init);
 
-  void append(ByteString name, ByteString value);
-  void delete(ByteString name);
+  undefined append(ByteString name, ByteString value);
+  undefined delete(ByteString name);
   ByteString? get(ByteString name);
   boolean has(ByteString name);
-  void set(ByteString name, ByteString value);
+  undefined set(ByteString name, ByteString value);
   iterable<ByteString, ByteString>;
 };
 </pre>