add remove instruction

instruction.html

@@ -60,7 +60,9 @@ <h5>Steps</h5>
  <li>Connect the events with the correct label based on the information from articles and
  paragraphs.<br>
  PS: Just drag the dot near the source event to the dot near the target event, and the
- connection will be built automatically.
+ connection will be built automatically.<br>
+ PS: If you want to remove the connection, you can click the "X" button near any end of the
+ connection, it'll remove all connections about that dot.
  </li>
  <li>Switch to the next pair until the end, and submit.</li>
  </ol>