Update screen-tracking.md

Author: ethanneff

versioned_docs/version-6.x/screen-tracking.md

@@ -32,6 +32,7 @@ export default () => {
       ref={navigationRef}
       onReady={() => {
         routeNameRef.current = navigationRef.getCurrentRoute().name;
+        // Replace the line below to add the tracker from a mobile analytics SDK
         await trackScreenView(routeNameRef.current);
       }}
       onStateChange={async () => {