doc: fix the return type of outgoingMessage.setHeaders()

The actual implementation returns `outgoingMessage` itself, but not
exactly `http.ServerResponse`.

Refs: https://github.com/nodejs/node/blob/20d8b85d3493bec944de541a896e0165dd356345/lib/_http_outgoing.js#L712-L751
PR-URL: #55290
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Qingyu Deng <i@ayase-lab.com>

doc/api/http.md

@@ -3289,9 +3289,7 @@ added:
 -->
 
 * `headers` {Headers|Map}
-* Returns: {http.ServerResponse}
-
-Returns the response object.
+* Returns: {this}
 
 Sets multiple header values for implicit headers.
 `headers` must be an instance of [`Headers`][] or `Map`,
@@ -3300,14 +3298,14 @@ its value will be replaced.
 
 ```js
 const headers = new Headers({ foo: 'bar' });
-response.setHeaders(headers);
+outgoingMessage.setHeaders(headers);
 ```
 
 or
 
 ```js
 const headers = new Map([['foo', 'bar']]);
-res.setHeaders(headers);
+outgoingMessage.setHeaders(headers);
 ```
 
 When headers have been set with [`outgoingMessage.setHeaders()`][],