bpf: Fix bpf_skc_lookup comment wrt. return type

The function no longer returns 'unsigned long' as of commit edbf8c01
("bpf: add skc_lookup_tcp helper").

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220617152121.29617-1-tklauser@distanz.ch