YAML API: fix default value for ethernet.auto-negotiation

Signed-off-by: Gris Ge <fge@redhat.com>

Author: cathay4t

devel/yaml_api.md

@@ -948,8 +948,7 @@ This property is ignored when applying if `auto-negotiation: true`.
 The `auto-negotiation` property is the boolean switch for enabling
 auto negotiation on speed and duplex.
 
-Default to `true`.
-
+Undefined means preserving current setting.
 
 ### SR-IOV