make Step 4 optional

Author: inventarSarah

docs/platforms/javascript/guides/react/index.mdx

@@ -160,7 +160,7 @@ import * as Sentry from "@sentry/react";
 </Alert>
 
 <OnboardingOption optionId="performance">
-## Step 4: Set Up React Router
+## Step 4: Set Up React Router (Optional)
 
 The React Router integration is designed to work with our tracing package. Learn more about set up for our [React Router Integration](configuration/integrations/react-router/).