Update docs/index.md

Co-authored-by: Christopher Hiller <boneskull@boneskull.com>

docs/index.md

@@ -690,7 +690,7 @@ function add(...args) {
   return args.reduce((prev, curr) => prev + curr, 0);
 }
 
-describe('add()', () => {
+describe('add()', function() {
   const tests = [
     {args: [1, 2], expected: 3},
     {args: [1, 2, 3], expected: 6},