doc: correct stream Duplex allowHalfOpen doc

If allowHalfOpen is set to false, the stream will automatically end the
writable side when the readable side ends, but not the other way around.

Fixes: nodejs#4044

doc/api/stream.md

@@ -1744,8 +1744,8 @@ constructor and implement *both* the `readable._read()` and
 * `options` {Object} Passed to both Writable and Readable
   constructors. Also has the following fields:
   * `allowHalfOpen` {boolean} Defaults to `true`. If set to `false`, then
-    the stream will automatically end the readable side when the
-    writable side ends and vice versa.
+    the stream will automatically end the writable side when the
+    readable side ends.
   * `readableObjectMode` {boolean} Defaults to `false`. Sets `objectMode`
     for readable side of the stream. Has no effect if `objectMode`
     is `true`.