yolov3 calculate the update of recall

After darknet compiles and calculates the recall, IoU
finds that the recall is too low
. Update detector.c is found on github

Original link: fix error in validate_detector_recall # 952

box t = {truth[j].x, truth[j].y, truth[j].w, truth[j].h};
            float best_iou = 0;
            // for(k = 0; k < l.w*l.h*l.n; ++k){
            for(k = 0; k < nboxes; ++k){  //用这个
                float iou = box_iou(dets[k].bbox, t);
                if(dets[k].objectness > thresh && iou > best_iou){
                    best_iou = iou;

Before

modification: After modification: