Fix Notebook 04 for invoke_async with context variables (#672)

### Motivation and Context
New Python API changes the signature for invoke_async.

This aligns it and makes it clearer that the context passed into
invoke_async are context variables.

This fixes the current issue where the chat function will just repeat
back the user input instead of actually chatting

---------

Co-authored-by: Abby Harrison <54643756+awharrison-28@users.noreply.github.com>

samples/notebooks/python/04-context-variables-chat.ipynb

@@ -108,8 +108,8 @@
       "metadata": {},
       "outputs": [],
       "source": [
-        "context = sk.ContextVariables()\n",
-        "context[\"history\"] = \"\""
+        "context_variables = sk.ContextVariables()\n",
+        "context_variables[\"history\"] = \"\""
       ]
     },
     {
@@ -127,8 +127,8 @@
       "metadata": {},
       "outputs": [],
       "source": [
-        "context[\"user_input\"] = \"Hi, I'm looking for book suggestions\"\n",
-        "bot_answer = await chat_function.invoke_async(input=context)\n",
+        "context_variables[\"user_input\"] = \"Hi, I'm looking for book suggestions\"\n",
+        "bot_answer = await chat_function.invoke_async(variables=context_variables)\n",
         "print(bot_answer)"
       ]
     },
@@ -147,8 +147,8 @@
       "metadata": {},
       "outputs": [],
       "source": [
-        "context[\"history\"] += f\"\\nUser: {context['user_input']}\\nChatBot: {bot_answer}\\n\"\n",
-        "print(context[\"history\"])"
+        "context_variables[\"history\"] += f\"\\nUser: {context_variables['user_input']}\\nChatBot: {bot_answer}\\n\"\n",
+        "print(context_variables[\"history\"])"
       ]
     },
     {
@@ -169,16 +169,16 @@
         "async def chat(input_text: str) -> None:\n",
         "    # Save new message in the context variables\n",
         "    print(f\"User: {input_text}\")\n",
-        "    context[\"user_input\"] = input_text\n",
+        "    context_variables[\"user_input\"] = input_text\n",
         "\n",
         "    # Process the user message and get an answer\n",
-        "    answer = await chat_function.invoke_async(context)\n",
+        "    answer = await chat_function.invoke_async(variables=context_variables)\n",
         "\n",
         "    # Show the response\n",
         "    print(f\"ChatBot: {answer}\")\n",
         "\n",
         "    # Append the new interaction to the chat history\n",
-        "    context[\"history\"] += f\"\\nUser: {input_text}\\nChatBot: {answer}\\n\""
+        "    context_variables[\"history\"] += f\"\\nUser: {input_text}\\nChatBot: {answer}\\n\""
       ]
     },
     {
@@ -243,4 +243,4 @@
   },
   "nbformat": 4,
   "nbformat_minor": 5
-}
+}