utilities: Wrong command syntax in ovs-vsctl manpage.

utilities: Wrong command syntax in ovs-vsctl manpage.

The command shown in the man page to disable the STP protocol on a bridge
is:

        ovs-vsctl clear Bridge br0 stp_enable

Calling that, the following warning message is returned:

        ovs-vsctl: "clear" operation cannot be applied to column stp_enable
        of table Bridge, which is not allowed to be empty

It seems correct to use the command:

        ovs-vsctl set Bridge br0 stp_enable=false

Signed-off-by: Daniele Venturino <daniele.venturino@m3s.it>
Signed-off-by: Ben Pfaff <blp@nicira.com>