Update instructions how to update AirSim in a custom environment

In particular, use the --delete flag with rsync, otherwise old source files
might be lying around that Unreal's build system would then try to pick up and
probably fail to build.

build.sh

@@ -122,7 +122,7 @@ echo " AirSim plugin is built! Here's how to build Unreal project."
 echo "=================================================================="
 echo "If you are using Blocks environment, its already updated."
 echo "If you are using your own environment, update plugin using,"
-echo "rsync -t -r Unreal/Plugins path/to/MyUnrealProject"
+echo "rsync -a --delete Unreal/Plugins path/to/MyUnrealProject"
 echo ""
 echo "For help see:"
 echo "https://github.com/Microsoft/AirSim/blob/master/docs/build_linux.md"