[docs] Fix "Enumerations and internal types" example

[crcrpar]

Description

This is a patch for the example mentioned in the title to be built because when building the snippet, I got the build error which includes:

    src/main.cpp:71:60: error: expected ';' at end of declaration
      py::class_<Pet::Attributes> attributes(pet, "Attributes")
                                                               ^
                                                               ;
    1 error generated.

Could you kindly let me know if there's a better change?

[rwgk]

Thanks! Looks good. I'm just waiting for the CI to finish. (I cannot imagine what could go wrong, but it's pretty quick.)