update symbol key for optional chaining and nullish coalescing

Author: keithamus

docs/index.html

@@ -726,7 +726,7 @@ <h1>GitHub Feature Support Table</h1>
                 <code>Optional Chaining Operator (?.)</code>
               </a>
             </th>
-            <td data-code="window?.foo?.bar?.baz || true"><div>**</div></td>
+            <td data-code="window?.foo?.bar?.baz || true"><div>!</div></td>
             <td data-supported="true"><div>80+</div></td>
             <td data-supported="true"><div>80+</div></td>
             <td data-supported="true"><div>74+</div></td>
@@ -740,7 +740,7 @@ <h1>GitHub Feature Support Table</h1>
                 <code>Nullish Coalescing Operator (??)</code>
               </a>
             </th>
-            <td data-code="window.foo ?? true"><div>**</div></td>
+            <td data-code="window.foo ?? true"><div>!</div></td>
             <td data-supported="true"><div>80+</div></td>
             <td data-supported="true"><div>80+</div></td>
             <td data-supported="true"><div>72+</div></td>