Update sitl_multiple_run.sh (PX4#21538)

Edited line to account for the fact that the file has been moved one level deeper in the folder tree (the same edit should be made in the documentation https://docs.px4.io/main/en/simulation/multi_vehicle_jmavsim.html)

Tools/simulation/sitl_multiple_run.sh

@@ -10,7 +10,7 @@ sitl_num=2
 [ -n "$1" ] && sitl_num="$1"
 
 SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
-src_path="$SCRIPT_DIR/.."
+src_path="$SCRIPT_DIR/../../"
 
 build_path=${src_path}/build/px4_sitl_default