Update get_signed_url for conversation.py (elevenlabs#414)

Was running into an error when using the Python convAI example code -> AttributeError: 'ElevenLabs' object has no attribute 'get_signed_url'  

Bug reported by user: elevenlabs#413 

This change seems to fix it.

Author: OsKrg

src/elevenlabs/conversational_ai/conversation.py

@@ -229,5 +229,5 @@ def _get_wss_url(self):
         return f"{base_ws_url}/v1/convai/conversation?agent_id={self.agent_id}"
 
     def _get_signed_url(self):
-        response = self.client.get_signed_url(agent_id=self.agent_id)
+        response = self.client.conversational_ai.get_signed_url(agent_id=self.agent_id)
         return response.signed_url