bpf, tests: Fix spelling mistake "shoft" -> "shift"

There is a spelling mistake in a literal string. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210815213950.47751-1-colin.king@canonical.com