Don't use '，' (U+FF0C) but ', ' (U+2C U+20) (#8063)

The Unicode codepoint '，' (U+FF0C) is meant to be used in Chinese text,
but this is English text. It looks like a comma followed by a space, but
isn't. Of course I didn't change actual Chinese text.

### What problem does this PR solve?

Mixup of Unicode characters. This is probably unnoticed by most users,
but I wonder if screen readers would read it out differently or if LLMs
would trip up on it.

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)
- [ ] New Feature (non-breaking change which adds functionality)
- [x] Documentation Update
- [ ] Refactoring
- [ ] Performance Improvement
- [ ] Other (please describe):

sdk/python/ragflow_sdk/modules/session.py

@@ -23,7 +23,7 @@ class Session(Base):
     def __init__(self, rag, res_dict):
         self.id = None
         self.name = "New session"
-        self.messages = [{"role": "assistant", "content": "Hi! I am your assistant，can I help you?"}]
+        self.messages = [{"role": "assistant", "content": "Hi! I am your assistant, can I help you?"}]
         for key, value in res_dict.items():
             if key == "chat_id" and value is not None:
                 self.chat_id = None
@@ -100,7 +100,7 @@ class Session(Base):
 
 class Message(Base):
     def __init__(self, rag, res_dict):
-        self.content = "Hi! I am your assistant，can I help you?"
+        self.content = "Hi! I am your assistant, can I help you?"
         self.reference = None
         self.role = "assistant"
         self.prompt = None