bpf: fix argument type in bpf_loop documentation

The `index` argument to bpf_loop() is threaded as an u64.
This lead in a subtle verifier denial where clang cloned the argument
in another register[1].

[1] https://github.com/systemd/systemd/pull/34650#issuecomment-2401092895



Signed-off-by: Matteo Croce <teknoraver@meta.com>
Link: https://lore.kernel.org/r/20241010035652.17830-1-technoboy85@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>