docs(docs): clarify LRUCache insertion order in example MDX

apps/docs/content/(hash-table)/146-lru-cache.mdx

@@ -47,10 +47,11 @@ class LRUCache:
         next_node.prev = prev_node
 
     def _add_node(self, node):
-        node.prev = self.head
-        node.next = self.head.next
-        self.head.next.prev = node  # pyright: ignore[reportAttributeAccessIssue]
-        self.head.next = node
+        # Insert just before the tail (most recent at end)
+        node.next = self.tail
+        node.prev = self.tail.prev
+        self.tail.prev.next = node  # pyright: ignore[reportAttributeAccessIssue]
+        self.tail.prev = node
 
     def get(self, key: int) -> int:
         if key not in self.cache:
@@ -73,7 +74,7 @@ class LRUCache:
             self._add_node(new_node)
 
             if len(self.cache) > self.capacity:
-                lru = self.tail.prev
+                lru = self.head.next  # Oldest node is next to the head
                 self._remove_node(lru)
                 del self.cache[lru.key]  # pyright: ignore[reportAttributeAccessIssue]