CBQE-0: minor fix in membase_upgrade_win

Change-Id: Ide543860ec70c7c14819df1b79c302e77b70c880
Reviewed-on: http://review.couchbase.org/27018
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>