doc: FVP to spawn tmux windows instead of xterm

Kronos FVP configuration in meta-arm layer has been modified to spawn
tmux windows instead of xterm to console into fvp cores. As a result,
the execution method of runfvp needs to be modified to accommodate this
change.

Signed-off-by: Vineeth Raveendran <vineeth.raveendran@arm.com>