Fixes a default config bug of gc scheduler

Signed-off-by: Yanqiang Miao <miao.yanqiang@zte.com.cn>

gc/scheduler/scheduler.go

@@ -91,6 +91,10 @@ func (d *duration) UnmarshalText(text []byte) error {
 	return nil
 }
 
+func (d duration) MarshalText() (text []byte, err error) {
+	return []byte(time.Duration(d).String()), nil
+}
+
 func init() {
 	plugin.Register(&plugin.Registration{
 		Type: plugin.GCPlugin,