[AIRFLOW-3782] Clarify docs around celery worker_autoscale in default_airflow.cfg (#4609)
[AIRFLOW-3782] Clarify docs around celery worker_autoscale in default_airflow.cfg (#4609)

Celery supports `autoscale` by accepting values in format "max_concurrency,min_concurrency".
But the default value in default_airflow.cfg is wrong, and the comment can be clearer.
danielvdende/incubator-airflow,wileeam/airflow,wileeam/airflow,nathanielvarona/airflow,wileeam/airflow,Fokko/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,r39132/airflow,mistercrunch/airflow,bolkedebruin/airflow,lyft/incubator-airflow,spektom/incubator-airflow,spektom/incubator-airflow,apache/incubator-airflow,airbnb/airflow,mtagle/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,sekikn/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,r39132/airflow,wooga/airflow,r39132/airflow,fenglu-g/incubator-airflow,mtagle/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,fenglu-g/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,mtagle/airflow,bolkedebruin/airflow,apache/airflow,apache/incubator-airflow,DinoCow/airflow,DinoCow/airflow,apache/incubator-airflow,airbnb/airflow,wooga/airflow,cfei18/incubator-airflow,mistercrunch/airflow,sekikn/incubator-airflow,apache/airflow,fenglu-g/incubator-airflow,bolkedebruin/airflow,artwr/airflow,artwr/airflow,wooga/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,spektom/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,Fokko/incubator-airflow,wooga/airflow,apache/airflow,Fokko/incubator-airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,lyft/incubator-airflow,apache/airflow,Fokko/incubator-airflow,apache/airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,wileeam/airflow,mistercrunch/airflow,nathanielvarona/airflow,mtagle/airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,artwr/airflow,nathanielvarona/airflow,apache/incubator-airflow,nathanielvarona/airflow,fenglu-g/incubator-airflow,artwr/airflow,dhuang/incubator-airflow,dhuang/incubator-airflow,r39132/airflow,DinoCow/airflow,DinoCow/airflow,lyft/incubator-airflow,spektom/incubator-airflow,mrkm4ntr/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow