fix(cpp-convention): Add namespace x::y to example

CPP_CODE_CONVENTION.md

@@ -187,8 +187,7 @@ than to read.
 // Header file commentary
 #pragma once
 
-namespace foo {
-namespace bar {
+namespace foo::bar {
 
 // When forward declaring a class you can choose to use indenting.
 namespace robot {
@@ -207,8 +206,7 @@ private:
     int the_answer_;
 };
 
-} // namespace bar
-} // namespace foo
+} // namespace foo::bar
 
 #endif
 ```