doc: fix `added:` info for `stream._construct()`

`readable._construct()` and `writable._construct()` were added to
Node.js v15.0.0 via fb8cc72e.

Fixes: https://github.com/nodejs/node/issues/36058

PR-URL: https://github.com/nodejs/node/pull/36067


Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>