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

`readable._construct()` and `writable._construct()` were added to
Node.js v15.0.0 via fb8cc72.

Fixes: #36058

PR-URL: #36067
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/stream.md

@@ -1981,7 +1981,7 @@ const myWritable = new Writable({
 
 #### `writable._construct(callback)`
 <!-- YAML
-added: v14.13.1
+added: v15.0.0
 -->
 
 * `callback` {Function} Call this function (optionally with an error
@@ -2299,7 +2299,7 @@ const myReadable = new Readable({
 
 #### `readable._construct(callback)`
 <!-- YAML
-added: v14.13.1
+added: v15.0.0
 -->
 
 * `callback` {Function} Call this function (optionally with an error