selftests/powerpc/pmu: fix spelling mistake "mis-match" -> "mismatch"

There are a few spelling mistakes in error messages. Fix them.

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/20220319232025.22067-1-colin.i.king@gmail.com