bpf, doc: Correct one wrong value in "Register value tracking"

If we then OR this with 0x40, then the value of 6th bit (0th is first bit)
become known, so the right mask is 0xbf instead of 0xcf.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>