[doc] Change OVN agent "extension" section to "agent"

This configuration variable is added here: [1].

[1]https://github.com/openstack/neutron/blob/cc05e169165f642a9096dfd09e35f5da9851dc24/neutron/conf/agent/agent_extensions_manager.py#L25

Signed-off-by: Rodolfo Alonso Hernandez <[email protected]>
Change-Id: Iecac0b628161412bc3bf04b28963a8f8f04ef0b9
(cherry picked from commit 9c079e5)

Author: ralonsoh

doc/source/ovn/ovn_agent.rst

@@ -38,7 +38,7 @@ agent configuration file in the "extensions" parameter:
 
 .. code-block:: console
 
-    [DEFAULT]
+    [agent]
     extensions = metadata