doc: Update INSTALL.Docker.md to reflect it's focus on OVN

doc: Update INSTALL.Docker.md to reflect it's focus on OVN

While reading this document, the title stood out to me as not
accurate. The title indicates it will discuss how to use
Open vSwitch with Docker, but in reality, it's about using
Open Virtual Networking with Docker.

This change updates the title, as well as the opening paragraphs
to more accurately reflect what the document is talking about.

Signed-off-by: Kyle Mestery <mestery@mestery.com>
Acked-by: Ryan Moats <rmoats@us.ibm.com>
Signed-off-by: Gurucharan Shetty <guru@ovn.org>