Update displayOperands to handle optional operands

The displayOperands function previously did not distinguish between optional and required operands.
This change updates the function to display optional operands within square brackets.

`vd,vs2,vs1,[,vm]`

src/App.js

@@ -31,12 +31,19 @@ class App extends Component {
     displayOperands(operands) {
         let all = "";
         let comma = "";
+
         for (let i = 0; i < operands.length; i++) {
-            all += comma + operands[i].name;
+            if (operands[i].optional) {
+                all += comma + "[," + operands[i].name + "]";
+            } else {
+                all += comma + operands[i].name;
+            }
             comma = ",";
         }
+
         return all;
     }
+
 
     displayMnemonic(item) {
         const spaces = "               ";