Update Set-MailboxCalendarConfiguration.md

A bit ambiguous here, we state that 'True' is the default value, and then we state that it is 'None'.
I checked the actual attribute and can confirm the real default value is indeed 'True'.

Author: Luspin

exchange/exchange-ps/exchange/client-access/Set-MailboxCalendarConfiguration.md

@@ -764,7 +764,7 @@ Aliases:
 Applicable: Exchange Server 2016, Exchange Online
 Required: False
 Position: Named
-Default value: None
+Default value: True
 Accept pipeline input: False
 Accept wildcard characters: False
 ```