board: nsim: doc: fix typo in platform name

Fix typo in nsim_hs6x_smp_12cores platform name (as previously
we had nsim_hs5x_smp_12cores platform mentioned two times)

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>