Nest some more namespaces

Did someone previously suggest that all namespace had been
nested?  Silly.

Signed-off-by: Stephan Pleines <pleines.stephan@gmail.com>
Change-Id: I22278c1caaeba8b47dc46fb2ed3078c20a11e190
diff --git a/y2023/vision/yolov5.h b/y2023/vision/yolov5.h
index 9853b4f..2d7f641 100644
--- a/y2023/vision/yolov5.h
+++ b/y2023/vision/yolov5.h
@@ -13,8 +13,7 @@
 #include <opencv2/imgcodecs.hpp>
 #include <opencv2/imgproc.hpp>
 
-namespace y2023 {
-namespace vision {
+namespace y2023::vision {
 
 struct Detection {
   cv::Rect box;
@@ -36,7 +35,6 @@
 
 std::unique_ptr<YOLOV5> MakeYOLOV5();
 
-}  // namespace vision
-}  // namespace y2023
+}  // namespace y2023::vision
 
 #endif  // Y2023_VISION_YOLOV5_H_