doc: fix transform stream example

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

PR-URL: #40777
Reviewed-By: Robert Nagy <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Rich Trott <[email protected]>

Author: evanlucas

doc/api/stream.md

@@ -3142,6 +3142,7 @@ pipeline(
         // Make sure is valid json.
         JSON.parse(this.data);
         this.push(this.data);
+        callback();
       } catch (err) {
         callback(err);
       }