Update README.md (#3986)

Update link to Grant Entitlement
https://www.revenuecat.com/docs/dashboard-and-metrics/customer-history/promotionals

### Description

Update README file

README.md

@@ -14,7 +14,7 @@ Whether you are building a new app or already have millions of customers, you ca
   * Fetch products, make purchases, and check subscription status with our [native SDKs](https://docs.revenuecat.com/docs/installation). 
   * Host and [configure products](https://docs.revenuecat.com/docs/entitlements) remotely from our dashboard. 
   * Analyze the most important metrics for your app business [in one place](https://docs.revenuecat.com/docs/charts).
-  * See customer transaction histories, chart lifetime value, and [grant promotional subscriptions](https://docs.revenuecat.com/docs/customers).
+  * See customer transaction histories, chart lifetime value, and [grant promotional subscriptions](https://www.revenuecat.com/docs/dashboard-and-metrics/customer-history/promotionals).
   * Get notified of real-time events through [webhooks](https://docs.revenuecat.com/docs/webhooks).
   * Send enriched purchase events to analytics and attribution tools with our easy integrations.