Fix minor typo (probably wrong reference)

[MiWeiss]

Pretty sure this change is what the author of the documentation intended (although, of course, the version currently on main is not wrong either) ;-)

Thanks heaps for open-sourcing voyager and for the very nice documentation.

[stephen29xie]

Hi, thanks for reporting this. That appears to be a typo, it should be Index.add_item.

The way to update documentation is by generating the docs based on select source files. For this typo it is in bindings.cpp.

https://github.com/spotify/voyager/blob/main/CONTRIBUTING.md#updating-documentation has more info. I haven't been able to run that Python script consistently though. #92 is migrating the Python bindings from pybind11 to Nanobind, which should remove the need for that script once it is merged.