Update min SDK in docs

Author: aurimasc

com.unity.mobile.notifications/Documentation~/index.md

@@ -5,7 +5,7 @@ The Unity Mobile Notifications package adds support for scheduling local one-tim
 ### Requirements
 
 - Compatible with Unity 2019.4 or above.
-- Compatible with Android 4.4+ (API 19) and iOS 10.0+.
+- Compatible with Android 5 (API 21) and iOS 10.0+.
 
 ### Supported features