add info about lab3 challenge 1

labcodes_answer/lab3_result/kern/trap/trap.c

@@ -183,7 +183,7 @@ trap_dispatch(struct trapframe *tf) {
         break;
     case IRQ_OFFSET + IRQ_TIMER:
 #if 0
-    
LAB3 : If some page replacement algorithm(such as CLOCK PRA) need tick to change the priority of pages, 
+    LAB3  CHALLENGEpan> 1 : If some page replacement algorithm(such as CLOCK PRA) need tick to change the priority of pages, 
     then you can add code here. 
 #endif
         /* LAB1 YOUR CODE : STEP 3 */