feat: add missing mark-read tooltip (#406)

Author: codebytere

src/js/components/__snapshots__/notification.test.tsx.snap

@@ -59,6 +59,7 @@ exports[`components/notification.js should render itself & its children 1`] = `
     <button
       className="sc-AxgMl beGRRX"
       onClick={[Function]}
+      title="Mark as Read"
     >
       <svg
         aria-hidden="false"

src/js/components/notification.tsx

@@ -116,7 +116,7 @@ export const NotificationItem: React.FC<IProps> = (props) => {
         </Details>
       </Main>
       <IconWrapper>
-        <Button onClick={() => markAsRead()}>
+        <Button title="Mark as Read" onClick={() => markAsRead()}>
           <Octicon icon={Check} size={20} ariaLabel="Mark as Read" />
         </Button>
       </IconWrapper>