docs: pmu: fix Unmatched example typo

bitmap for MHPMCOUNTERx should be 0x18 and not 0x0c, we check
against SBI_PMU_FIXED_CTR_MASK which assumes than first 3 bits are
dedicated to mcycle, mtime and minstret, u74 has 2 hardware counters.

Reported-by: Zhang Xin <zhangxin.xa@gmail.com>
Signed-off-by: Nikita Shubin <n.shubin@yadro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>