Change the operations sequence of the likedlist's insert() method.

codes/zig/chapter_array_and_linkedlist/linked_list.zig

@@ -8,8 +8,8 @@ const inc = @import("include");
 // 在链表的结点 n0 之后插入结点 P
 pub fn insert(n0: ?*inc.ListNode(i32), P: ?*inc.ListNode(i32)) void {
     var n1 = n0.?.next;
-    n0.?.next = P;
     P.?.next = n1;
+    n0.?.next = P;
 }
 
 // 删除链表的结点 n0 之后的首个结点