Fix typo in doc docker-stack (#14928)
Fix typo in doc docker-stack (#14928)

Fix a typo on doc , the _AIRFLOW_WWW_USER_PASSWORD_CMD is repeated two times
cfei18/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,apache/airflow,mistercrunch/airflow,mistercrunch/airflow,cfei18/incubator-airflow,mistercrunch/airflow,dhuang/incubator-airflow,mistercrunch/airflow,apache/incubator-airflow,apache/airflow,lyft/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,apache/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,apache/airflow,nathanielvarona/airflow,nathanielvarona/airflow,sekikn/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,apache/airflow