selftests/powerpc: Fix spelling mistake "mmaping" -> "mmapping"

There is a spelling mistake in a perror 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://lore.kernel.org/r/20221021084545.65973-1-colin.i.king@gmail.com