selftests/bpf: Fix spelling mistake "scoket" -> "socket"

There is a spelling mistake in a check error message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200826085907.43095-1-colin.king@canonical.com