doc: fix default of duplex.allowHalfOpen

PR-URL: #43665
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

Author: vweevers

doc/api/stream.md

@@ -2270,7 +2270,7 @@ added: v0.9.4
 
 If `false` then the stream will automatically end the writable side when the
 readable side ends. Set initially by the `allowHalfOpen` constructor option,
-which defaults to `false`.
+which defaults to `true`.
 
 This can be changed manually to change the half-open behavior of an existing
 `Duplex` stream instance, but must be changed before the `'end'` event is