Fix misleading statement on sqlite (#14317)
Fix misleading statement on sqlite (#14317)

The statement 
```
By default, Airflow uses **SQLite**, which is *not* intended for development purposes only.
```
is confusing. If `postgres/mysql` are production worthy db backends, and `sqlite` as default db for `airflow` is for development purposes only, this statement is not correct. If I'm mistaken and `sqlite` is for both `production` and `development` purposes, please ignore this PR
apache/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,mistercrunch/airflow,apache/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,mistercrunch/airflow,apache/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,sekikn/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,apache/airflow,apache/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,apache/incubator-airflow