Fix typo in MySQL Database creation code (Set up DB docs)  (#20102)
Fix typo in MySQL Database creation code (Set up DB docs)  (#20102)

* Fix typo in MySQL Database creation code

Character set `utf8` is an alias for `utf8mb3`, see docs linked below. This means collation should be set to `utf8mb3_unicode_ci`.
Using `COLLATE utf8mb4_unicode_ci` (current code) throws the following error:
`ERROR 1253 (42000): COLLATION 'utf8mb4_unicode_ci' is not valid for CHARACTER SET 'utf8'`
apache/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,apache/airflow,cfei18/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,apache/airflow,nathanielvarona/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,apache/airflow,nathanielvarona/airflow,nathanielvarona/airflow,mistercrunch/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow