libbpf: Fix trivial typo

Fix typo in comment from 'bpf_skeleton_map' to 'bpf_map_skeleton'
and from 'bpf_skeleton_prog' to 'bpf_prog_skeleton'.

Signed-off-by: huangxuesen <huangxuesen@kuaishou.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1638755236-3851199-1-git-send-email-hxseverything@gmail.com