Update 04_Slider.js
Update 04_Slider.js

the original looks ok on Chrome latest, but not on Firefox latest. my proposed change looks good in both
chellyjin/p5.js-website,processing/p5.js-website,lasyapriyanka/p5.js-website,lasyapriyanka/p5.js-website,processing/p5.js-website,chellyjin/p5.js-website