tailcfg: fix stale docs for MapResponse.KeepAlive
@@ -1195,7 +1195,7 @@ type PingRequest struct {
type MapResponse struct {
// KeepAlive, if set, represents an empty message just to keep
// the connection alive. When true, all other fields except
- // PingRequestURL are ignored.
+ // PingRequest are ignored.
KeepAlive bool `json:",omitempty"`
// PingRequest, if non-empty, is a request to the client to
