Update GROK_API_KEY to XAI_API_KEY

examples/foundational/55zza-update-settings-grok-llm.py

@@ -60,7 +60,7 @@ async def run_bot(transport: BaseTransport, runner_args: RunnerArguments):
     )
 
     llm = GrokLLMService(
-        api_key=os.getenv("GROK_API_KEY"),
+        api_key=os.getenv("XAI_API_KEY"),
         settings=GrokLLMService.Settings(
             system_instruction="You are a helpful assistant in a voice conversation. Your responses will be spoken aloud, so avoid emojis, bullet points, or other formatting that can't be spoken. Respond to what the user said in a creative, helpful, and brief way.",
         ),