doc: update writable.write return value

Fix the explanation which stated that write() would return false if
highWaterMark is exceeded to correctly state that false is returned
once highWaterMark is reached. See nodejs#9247.

PR-URL: nodejs#10582
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

doc/api/stream.md

@@ -440,7 +440,7 @@ occurs, the `callback` *may or may not* be called with the error as its
 first argument. To reliably detect write errors, add a listener for the
 `'error'` event.
 
-The return value is `true` if the internal buffer does not exceed
+The return value is `true` if the internal buffer is less than the
 `highWaterMark` configured when the stream was created after admitting `chunk`.
 If `false` is returned, further attempts to write data to the stream should
 stop until the [`'drain'`][] event is emitted.