Typo in VPN script

[easyveazie]

https://github.com/Microsoft/sql-server-samples/blob/276a4474e30dc84f4a56bb5f170bbb98a403e6cc/samples/manage/azure-sql-db-managed-instance/attach-vpn-gateway/attachVPNGateway.ps1#L30

[mikepetrak]

Good catch, Mike Veazie (@easyveazie)! I fixed this typo (and another one close by) - see #548 .