'todo' -> 'TODO' refactoring

test/unit_test/static/custom_unit_test.cpp

@@ -67,7 +67,7 @@ namespace {
 
   // amplitude spectral density
   struct amplitude_spectral_density : derived_dimension<amplitude_spectral_density, units::exp<voltage, 1>, units::exp<frequency, -1, 2>> {};
-  // todo: add support for derived_unit
+  // TODO: add support for derived_unit
   //struct volt_per_sq_hertz : derived_unit<volt_per_sq_hertz, amplitude_spectral_density, volt, hertz> {};
   struct volt_per_sqrt_hertz : derived_unit<volt_per_sqrt_hertz, decltype("V/Hz^(1/2)"_fs), amplitude_spectral_density> {};
 }