Add unreleased changes to FirebaseAppDistribution/CHANGELOG.md (#10775)

Author: lfkellogg

FirebaseAppDistribution/CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- [fixed] Fixed bug where testers were sent to the wrong URL when signing in (#10772).
+
 # 10.2.0
 - [added] Added a public `application(_:openURL:options:)` method so users
   with swizzling disabled can still use App Distribution (#10418).