doc: shields: fix wrong board name for mimxrt1170_evk//cm7

fix a typo in board name (mixm... instead of mimx...) and actually use
a proper board name anyway since it looks like this was still using
a HWMv1 name.
Fixes zephyrproject-rtos/zephyr#95015

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>