Fixed incorrect usage of fmt, causing compile errors

[sprenger120]

Just found the library and I really like it. It saves so much time; thank you for your work. There is just a tiny error in the code generator, causing the following error

error: temporary of non-literal type ‘std::__cxx11::basic_string<char>’ in a constant expression
  256 |           throw rclcpp::exceptions::InvalidParameterValueException(fmt::format("Invalid value set during initialization for parameter 'led_strip.block_length': " + validation_result.error()));

The fix is easy, replace the "+ validation_result" with ", validation_result" and add a "{}" for placeholding. That is all this PR does

[sjahr]

I don't have issues building this package without this change but building it with this works too 👍 @pac48 Can you take a look at this?

[sprenger120]

@sjahr Did you try forcing C++20? I forgot to mention that

[tylerjw]

Thank you for this fix! I'm running CI to ensure it doesn't break anything we already tested for, but the change looks good. I will add an issue to add a test to force C++20 in one of the CI jobs so we can catch this in the future. I don't want to make it hard for people to use C++20 by accident.