Found typo in bigo.js

[RonSkufca]

Rob, I discovered a typo in sample-code/theory/bigo.js. The last line under Gauss's trick the variable listItem does not exist. I believe it should be lastItem.

const sumContiguousArray = function(ary){
  //get the last item
  const lastItem = ary[ary.length - 1];
  //Gauss's trick
  return lastItem * (listItem + 1) / 2;
}