Apply suggestions from code review

Co-authored-by: Jordan Harband <ljharb@gmail.com>

README.md

@@ -7,7 +7,7 @@ const array = [1, 2, 3, 4, 5];
 
 // `Object.groupBy` groups items by arbitrary key.
 // In this case, we're grouping by even/odd keys
-Object.groupBy(array, (num, index, array) => {
+Object.groupBy(array, (num, index) => {
   return num % 2 === 0 ? 'even': 'odd';
 });
 // =>  { odd: [1, 3, 5], even: [2, 4] }
@@ -16,7 +16,7 @@ Object.groupBy(array, (num, index, array) => {
 // using an object key.
 const odd  = { odd: true };
 const even = { even: true };
-Map.groupBy(array, (num, index, array) => {
+Map.groupBy(array, (num, index) => {
   return num % 2 === 0 ? even: odd;
 });
 // =>  Map { {odd: true}: [1, 3, 5], {even: true}: [2, 4] }