vswitchd.xml: Fix supported IPsec tunnels.
'ovs-monitor-ipsec' does not support 'ip6gre' tunnels.
Fixes: ("ipsec: reintroduce IPsec support for tunneling")
<group title="Tunnel Options: IPsec">
<p>
Setting any of these options enables IPsec support for a given
- tunnel. <code>gre</code>, <code>ip6gre</code>,
- <code>geneve</code>, <code>vxlan</code> and <code>stt</code>
+ tunnel. <code>gre</code>, <code>geneve</code>,
+ <code>vxlan</code> and <code>stt</code>
interfaces support these options. See the <code>IPsec</code>
section in the <ref table="Open_vSwitch"/> table for a description
of each mode.
