Update Set-CsTeamsFilesPolicy.md

Updating a setting which is currently in preview (available only in inner rings), based on the recommendations of Microsoft Security teams. This will be re-updated once the feature rollout is complete.

Author: apoorv33

teams/teams-ps/teams/Set-CsTeamsFilesPolicy.md

@@ -137,7 +137,7 @@ Accept wildcard characters: False
 
 ### -FileSharingInChatswithExternalUsers
 
-Indicates if file sharing in chats with external users is enabled.
+[Currently in Preview] Indicates if file sharing in chats with external users is enabled.
 
 ```yaml
 Type: String