Fixed Parameter Default Value

Author: nsshommo

exchange/exchange-ps/exchange/mailboxes/Add-MailboxPermission.md

@@ -281,7 +281,7 @@ Aliases:
 Applicable: Exchange Server 2010, Exchange Server 2013, Exchange Server 2016, Exchange Online
 Required: False
 Position: Named
-Default value: None
+Default value: $true
 Accept pipeline input: False
 Accept wildcard characters: False
 ```