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/y2020/vision/sift/sift971.h b/y2020/vision/sift/sift971.h
index b351d70..a235afb 100644
--- a/y2020/vision/sift/sift971.h
+++ b/y2020/vision/sift/sift971.h
@@ -6,8 +6,7 @@
 #include <opencv2/core/types.hpp>
 #include <opencv2/features2d.hpp>
 
-namespace frc971 {
-namespace vision {
+namespace frc971::vision {
 
 /*!
  SIFT implementation.
@@ -66,7 +65,6 @@
   bool use_fast_pyramid_difference_ = true;
 };
 
-}  // namespace vision
-}  // namespace frc971
+}  // namespace frc971::vision
 
 #endif  // Y2020_VISION_SIFT_SIFT971_H_