make ESlint happy

components/signUpsModal.jsx

@@ -24,7 +24,7 @@ export default function SignUpsModal({ open, close, event }) {
         </thead>
         <tbody>
           {event.signUps.map((signUp, i) => (
-            <tr>
+            <tr key={i}>
               <td>
                 {signUp.firstName} {signUp.lastName}
               </td>