Use C++17 format for nested namespace definitions

Use the new C++17 format for defining nested namespaces in the
phosphor-regulators application.

The definition
  namespace A::B::C
  {
  ...
  }

is now equivalent to
   namespace A
   {
   namespace B
   {
   namespace C
   {
   ...
   }
   }
   }

Signed-off-by: Shawn McCarney <shawnmm@us.ibm.com>
Change-Id: I284055bc94578a45d04d3561f511c0e347a8ecef
diff --git a/phosphor-regulators/src/rail.hpp b/phosphor-regulators/src/rail.hpp
index d5d3b13..247e8ee 100644
--- a/phosphor-regulators/src/rail.hpp
+++ b/phosphor-regulators/src/rail.hpp
@@ -17,11 +17,7 @@
 
 #include <string>
 
-namespace phosphor
-{
-namespace power
-{
-namespace regulators
+namespace phosphor::power::regulators
 {
 
 /**
@@ -69,6 +65,4 @@
     const std::string id{};
 };
 
-} // namespace regulators
-} // namespace power
-} // namespace phosphor
+} // namespace phosphor::power::regulators