Update linked_list.md (krahets#935)

There is no need to declare next

Author: jiaxianhua

docs/chapter_array_and_linkedlist/linked_list.md

@@ -152,7 +152,7 @@
 
     /* 构造函数 */
     ListNode *newListNode(int val) {
-        ListNode *node, *next;
+        ListNode *node;
         node = (ListNode *) malloc(sizeof(ListNode));
         node->val = val;
         node->next = NULL;
@@ -618,7 +618,7 @@
 
     /* 构造函数 */
     ListNode *newListNode(int val) {
-        ListNode *node, *next;
+        ListNode *node;
         node = (ListNode *) malloc(sizeof(ListNode));
         node->val = val;
         node->next = NULL;