fix doctest

Author: sycai

bigframes/bigquery/_operations/ai.py

@@ -60,7 +60,7 @@ def generate_bool(
         0    {'result': True, 'full_response': '{"candidate...
         1    {'result': True, 'full_response': '{"candidate...
         2    {'result': False, 'full_response': '{"candidat...
-        dtype: struct<result: bool, full_response: string, status: string>[pyarrow]
+        dtype: struct<result: bool, full_response: extension<dbjson<JSONArrowType>>, status: string>[pyarrow]
 
         >>> bbq.ai.generate_bool((df["col_1"], " is a ", df["col_2"])).struct.field("result")
         0     True