doc: fix transform stream example

There was a missing callback in the Transform#flush example
implementation.

PR-URL: #40777
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/stream.md

@@ -3230,6 +3230,7 @@ pipeline(
         // Make sure is valid json.
         JSON.parse(this.data);
         this.push(this.data);
+        callback();
       } catch (err) {
         callback(err);
       }