Update openvswitch.rst

Today I found out that not only "mac-spoofing" prevention filters are active, but also "IP hijacking" filters for IPv4. This should be reflected in the docs, would have saved me some time. Ideally IPv6 hijacking should also be implemented, I might right a small patch for that later. IP-aliases should be possible however, so all ip-adresses that _are_ configured are allowed. IP Hijack prevention is a great feature, especially for public clouds.
Merge pull request #4 from hydro-b/patch-1

Update openvswitch.rst