Return const-reference instead of value from read-accessor operator[]

[fknorr]

This becomes visible when trying to take the address of a buffer element.

[BlackMark29A]

Returning a value instead of a const-reference from an index operator seems strange to me in the first place. So unless there was a particular reason why this was done I would say changing it to const-reference is the correct thing to do.

[fknorr]

I believe it was simply a typo. SYCL also mandates references in the return type: https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#_interface_for_buffer_command_accessors