kselftest: arm64: fix spelling mistake "contiguos" -> "contiguous"

There is a spelling mistake in an error message literal string. Fix it.

Fixes: f96bf434

 ("kselftest: arm64: mangle_pstate_invalid_compat_toggle and common utils")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>