add easy_lab2 solution

Author: JiajunDu

homework.html

@@ -127,7 +127,7 @@ <h3>作业</h3>
                             <td>easy_lab2: Accelerate matrix multiplication</td>
                             <td>10月27日-11月17日 23:59</td>
                             <td>详见<a href="https://github.com/BUPT-OS/easy_lab/tree/lab2" style="color: #FF0000;">easy_lab/lab2</a></td>
-                            <td></td>
+                            <td><a href="./homework/easy_lab2.pdf" style="color: #FF0000;">solution.pdf</a></td>
                         </tr>
                         <tr style="background-color: lightgreen;">
                             <td>作业6</td>