Fix: truncate message issue. (#5776)

### What problem does this PR solve?

Close #5761
### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

rag/prompts.py

         msg[0]["content"] = m
         return max_length, msg
 
-    m = msg_[1]["content"]
+    m = msg_[-1]["content"]
     m = encoder.decode(encoder.encode(m)[:max_length - ll2])
-    msg[1]["content"] = m
+    msg[-1]["content"] = m
     return max_length, msg