update changelog.md

Author: isra-fel

src/ApiManagement/ApiManagement/ChangeLog.md

@@ -18,7 +18,7 @@
         - Additional information about change #1
 -->
 ## Upcoming Release
-* `New-AzApiManagementProduct`: Change SubscriptionsLimit parameter default value to None
+* [Breaking change] `New-AzApiManagementProduct` by default has no subscription limit.
 
 ## Version 2.1.0
 * Added new `Add-AzApiManagementApiToGateway` cmdlet.