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/get_gaussian_kernel.h b/y2020/vision/sift/get_gaussian_kernel.h
index c13c588..9252e24 100644
--- a/y2020/vision/sift/get_gaussian_kernel.h
+++ b/y2020/vision/sift/get_gaussian_kernel.h
@@ -4,8 +4,7 @@
 #include <cmath>
 #include <vector>
 
-namespace frc971 {
-namespace vision {
+namespace frc971::vision {
 
 // A reimplementation of cv::getGaussianKernel for CV_32F without external
 // dependencies. See fast_gaussian_halide_generator.sh for details why we want
@@ -24,7 +23,6 @@
   return result;
 }
 
-}  // namespace vision
-}  // namespace frc971
+}  // namespace frc971::vision
 
 #endif  // Y2020_VISION_SIFT_GET_GAUSSIAN_KERNEL_H_