Update multi-service_container.md

Negative exitcodes are not allowed, see:

http://tldp.org/LDP/abs/html/exitcodes.html

exit takes only integer args in the range 0 - 255

config/containers/multi-service_container.md

@@ -68,7 +68,7 @@ this in a few different ways.
     # If they are not both 0, then something is wrong
     if [ $PROCESS_1_STATUS -ne 0 -o $PROCESS_2_STATUS -ne 0 ]; then
       echo "One of the processes has already exited."
-      exit -1
+      exit 1
     fi
   done
   ```