docs/bpf: update BPF map definition example

Use BTF-defined map definition in the documentation example.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220120060529.1890907-5-andrii@kernel.org


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