doc: add missing `added:` tag for `zlib.constants`

PR-URL: https://github.com/nodejs/node/pull/10826
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/zlib.md

@@ -388,6 +388,9 @@ Reset the compressor/decompressor to factory defaults. Only applicable to
 the inflate and deflate algorithms.
 
 ## zlib.constants
+<!-- YAML
+added: v7.0.0
+-->
 
 Provides an object enumerating Zlib-related constants.