Update CHANGELOG.md (Azure#24454)

Author: maririos

sdk/formrecognizer/Azure.AI.FormRecognizer/CHANGELOG.md

@@ -1,6 +1,6 @@
 # Release History
 
-## 4.0.0-beta.1 (2021-10-05)
+## 4.0.0-beta.1 (Unreleased)
 > Note: Starting with version `2021-09-30-preview`, a new set of clients were introduced to leverage the newest features of the Form Recognizer service. Please see the [Migration Guide](https://github.com/Azure/azure-sdk-for-net/tree/main/sdk/formrecognizer/Azure.AI.FormRecognizer/MigrationGuide.md) for detailed instructions on how to update application code from client library version `3.1.X` or lower to the latest version.
 
 ### Features Added