docs: pi definition fixed

docs/users_guide/framework_basics/systems_of_units.md

@@ -196,7 +196,7 @@ For some units, a magnitude might also be irrational. The best example here is a
 is defined using a floating-point magnitude having a factor of the number π (Pi):
 
 ```cpp
-inline constexpr struct pi final : mag_constant<symbol_text{u8"π", "pi"}, std::numbers::pi_v<long double>> {
+inline constexpr struct pi final : mag_constant<symbol_text{u8"π", "pi"}, std::numbers::pi_v<long double>> {} pi;
 inline constexpr auto π = pi;
 ```