[Bug] fix requirements.txt for example chat-w/-pdf (#284)

* fix(chat-w/-pdf): fix requirements.txt for example

* fix(chat-w/-pdf): pin only explicitly imported packages

Author: LatentDream

examples/panel/chat-with-pdf/requirements.txt

@@ -1,6 +1,7 @@
-panel
-openai
-chromadb
-pypdf
-langchain
-tiktoken
+tiktoken==0.7.0
+langchain==0.1.11
+langchain-community==0.0.25
+pypdf==5.0.1
+chromadb==0.5.11
+openai==1.51.0
+panel==1.5.1