Chore: use engine_adapter.execute for statements in python model docs (#4754)

Author: georgesittas

docs/concepts/models/python_models.md

@@ -163,15 +163,15 @@ def execute(
 ) -> pd.DataFrame:
 
     # pre-statement
-    context.fetchdf("SET GLOBAL parameter = 'value';")
+    context.engine_adapter.execute("SET GLOBAL parameter = 'value';")
 
     # post-statement requires using `yield` instead of `return`
     yield pd.DataFrame([
         {"id": 1, "name": "name"}
     ])
 
     # post-statement
-    context.fetchdf("CREATE INDEX idx ON example.pre_post_statements (id);")
+    context.engine_adapter.execute("CREATE INDEX idx ON example.pre_post_statements (id);")
 ```
 
 ## Optional on-virtual-update statements