invalid yang files

Feature can`t be defined in augment, only in models root.

please see YANG snippet below:

module service-function-forwarder-metadata-features
.
.
augment "/sfc-sff:service-function-forwarders/" {
feature vxlan-classifier-type-1

{ description "Feature definition for C1: IPv4 tunnel dest and" + "C2: VNID"; }

description
"Augments SFF module with definition of" +
"VXLAN overlay classifier feature";
ext:augment-identifier "sff-vxlan-classifier-type-1-feature";
}