selftests/powerpc/dexcr: Fix spelling mistake "predicition" -> "prediction"

There is a spelling mistake in the help message. Fix it.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240508084117.2869261-1-colin.i.king@gmail.com