doc: add missing zlib link to stream API docs

This seems to have been removed inadvertently by
330c8d743e in PR 12925.

PR-URL: https://github.com/nodejs/node/pull/13838
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/stream.md

@@ -2060,6 +2060,7 @@ readable buffer so there is nothing for a user to consume.
 [HTTP responses, on the server]: http.html#http_class_http_serverresponse
 [http-incoming-message]: http.html#http_class_http_incomingmessage
 [Readable]: #stream_class_stream_readable
+[zlib]: zlib.html
 [stream-_flush]: #stream_transform_flush_callback
 [stream-_read]: #stream_readable_read_size_1
 [stream-_transform]: #stream_transform_transform_chunk_encoding_callback