Update buffer.mdx (langchain-ai#2123)

Deprecated imports of `import { HumanChatMessage, AIChatMessage } from "langchain/schema";`

Replaced with 
`import { AIMessage, HumanMessage } from "langchain/schema";`

Author: l4b4r4b4b4

docs/snippets/modules/memory/how_to/buffer.mdx

@@ -28,7 +28,7 @@ This lets you easily pick up state from past conversations:
 
 ```typescript
 import { BufferMemory, ChatMessageHistory } from "langchain/memory";
-import { HumanChatMessage, AIChatMessage } from "langchain/schema";
+import { HumanMessage, AIMessage } from "langchain/schema";
 
 const pastMessages = [
   new HumanMessage("My name's Jonas"),