refactor(example): simpler header files used

example/hello_units.cpp

@@ -28,8 +28,8 @@
 #include <mp-units/format.h>
 #include <mp-units/ostream.h>
 #include <mp-units/systems/international/international.h>
-#include <mp-units/systems/isq/space_and_time.h>
-#include <mp-units/systems/si/unit_symbols.h>
+#include <mp-units/systems/isq/isq.h>
+#include <mp-units/systems/si/si.h>
 #include <iostream>
 
 using namespace mp_units;