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/fast_gaussian.h b/y2020/vision/sift/fast_gaussian.h
index f243e53..d9a5f07 100644
--- a/y2020/vision/sift/fast_gaussian.h
+++ b/y2020/vision/sift/fast_gaussian.h
@@ -8,8 +8,7 @@
 
 #include "HalideBuffer.h"
 
-namespace frc971 {
-namespace vision {
+namespace frc971::vision {
 
 // Returns a Halide buffer representing the data in mat.
 template <typename T>
@@ -39,7 +38,6 @@
 void FastGaussianAndSubtract(const cv::Mat &source, cv::Mat *blurred,
                              cv::Mat *difference, double sigma);
 
-}  // namespace vision
-}  // namespace frc971
+}  // namespace frc971::vision
 
 #endif  // Y2020_VISION_SIFT_FAST_GAUSSIAN_H_