doc: improve doc for http.ServerResponse inheritance

Document that `http.ServerResponse` inherits from
`http.OutgoingMessage`.

PR-URL: nodejs/node#42693
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/http.md

@@ -1497,7 +1497,7 @@ affects new connections to the server, not any existing connections.
 added: v0.1.17
 -->
 
-* Extends: {Stream}
+* Extends: {http.OutgoingMessage}
 
 This object is created internally by an HTTP server, not by the user. It is
 passed as the second parameter to the [`'request'`][] event.