Minor improvement to ButtonsSlidersMenus demo program
Minor improvement to ButtonsSlidersMenus demo program

sspickle/vpython-jupyter,BruceSherwood/vpython-jupyter,BruceSherwood/vpython-jupyter,mwcraig/vpython-jupyter,mwcraig/vpython-jupyter,mwcraig/vpython-jupyter,mwcraig/vpython-jupyter,sspickle/vpython-jupyter,BruceSherwood/vpython-jupyter,BruceSherwood/vpython-jupyter