updating Understand-index.md

as of version 1.9.1 the index uses the .first method instead of .last
check the code in the .min.js attached `index:function(e){return e?"string"==typeof e?x.inArray(this[0],x(e)):x.inArray(e.jquery?e[0]:e,this):this[0]&&this[0].parentNode?this.first().prevAll().length:-1}`