Update app.py to switch to free tier model_id

app.py

@@ -30,7 +30,7 @@ class BasicAgent:
         model = HfApiModel(
             max_tokens=2096,
             temperature=0.0,
-            model_id="mistralai/Mistral-7B-Instruct-v0.2",# it is possible that this model may be overloaded
+            model_id="google/gemma-2b-it",# it is possible that this model may be overloaded
             custom_role_conversions=None,
         )