Fix multi query scenario in bigquery example DAG (#21575)

- The Google Bigquery Jobs api can accept multiple queries passed in
  a string separated by a semicolon. When queries are passed in a list,
  then it ends up executing first query only.

Author: phanikumv

airflow/providers/google/cloud/example_dags/example_bigquery_queries.py

@@ -138,10 +138,7 @@
             task_id="execute_multi_query",
             configuration={
                 "query": {
-                    "query": [
-                        f"SELECT * FROM {DATASET}.{TABLE_2}",
-                        f"SELECT COUNT(*) FROM {DATASET}.{TABLE_2}",
-                    ],
+                    "query": f"SELECT * FROM {DATASET}.{TABLE_2};SELECT COUNT(*) FROM {DATASET}.{TABLE_2}",
                     "useLegacySql": False,
                 }
             },