samples: Fix confusing openamp sample doc

A lot of users seem to be confused by this sample when they try to do
west debug. This is because Zephyr currently does not support flashing
all the cores from a sysbuild automatically when doing a west debug
command. So west flash needs to be done prior to a west debug command.
The README previously actually said the step by step instructions are to
do the build, then immediately west debug, which would cause faulting
and crashing, so change the instructions goal to be a flash, and put a
note about needing to flash before debugging.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>