Update New-CsTeamsFilesPolicy.md with disable command

Added command to disable file sharing in chats with external users.

Author: gramirez-msft

teams/teams-ps/MicrosoftTeams/New-CsTeamsFilesPolicy.md

@@ -52,7 +52,10 @@ Accept wildcard characters: False
 
 ### -FileSharingInChatswithExternalUsers
 
-Indicates if file sharing in chats with external users is enabled.
+Indicates if file sharing in chats with external users is enabled. It is by default enabled, to disable admins can run following command.
+```powershell
+Set-CsTeamsFilesPolicy -Identity Global -FileSharingInChatswithExternalUsers Disabled
+```
 
 ```yaml
 Type: String