Merge pull request #12746 from MaximeDelarue/patch-1

Update Add-MailboxPermission.md

Author: chrisda

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

@@ -420,7 +420,7 @@ Applicable: Exchange Server 2010, Exchange Server 2013, Exchange Server 2016, Ex
 
 Required: False
 Position: Named
-Default value: None
+Default value: All
 Accept pipeline input: False
 Accept wildcard characters: False
 ```