[AIRFLOW-3007] Update backfill example in Scheduler docs
[AIRFLOW-3007] Update backfill example in Scheduler docs

The scheduler docs at https://airflow.apache.org/scheduler.html#backfill-and-catchup use deprecated way of passing `schedule_interval`. `schedule_interval` should be pass to DAG as a separate parameter and not as a default arg.

jgao54/airflow,jgao54/airflow,Fokko/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,jgao54/airflow,sekikn/incubator-airflow,DinoCow/airflow,DinoCow/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,wooga/airflow,bolkedebruin/airflow,akosel/incubator-airflow,danielvdende/incubator-airflow,wileeam/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,fenglu-g/incubator-airflow,mtagle/airflow,dhuang/incubator-airflow,spektom/incubator-airflow,apache/airflow,DinoCow/airflow,r39132/airflow,adamhaney/airflow,Acehaidrey/incubator-airflow,wooga/airflow,jgao54/airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,akosel/incubator-airflow,nathanielvarona/airflow,fenglu-g/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,lyft/incubator-airflow,cfei18/incubator-airflow,wileeam/airflow,airbnb/airflow,cfei18/incubator-airflow,malmiron/incubator-airflow,adamhaney/airflow,apache/airflow,apache/incubator-airflow,spektom/incubator-airflow,sid88in/incubator-airflow,malmiron/incubator-airflow,Fokko/incubator-airflow,spektom/incubator-airflow,mrkm4ntr/incubator-airflow,mistercrunch/airflow,sid88in/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,wooga/airflow,malmiron/incubator-airflow,apache/airflow,spektom/incubator-airflow,mrkm4ntr/incubator-airflow,apache/airflow,andyxhadji/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,Fokko/incubator-airflow,airbnb/airflow,danielvdende/incubator-airflow,mrkm4ntr/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,apache/airflow,artwr/airflow,nathanielvarona/airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,apache/incubator-airflow,apache/incubator-airflow,Fokko/incubator-airflow,Acehaidrey/incubator-airflow,airbnb/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,apache/airflow,andyxhadji/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,lyft/incubator-airflow,andyxhadji/incubator-airflow,lyft/incubator-airflow,akosel/incubator-airflow,artwr/airflow,fenglu-g/incubator-airflow,sekikn/incubator-airflow,artwr/airflow,mtagle/airflow,Acehaidrey/incubator-airflow,adamhaney/airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,r39132/airflow,wileeam/airflow,andyxhadji/incubator-airflow,lyft/incubator-airflow,airbnb/airflow,adamhaney/airflow,sid88in/incubator-airflow,malmiron/incubator-airflow,fenglu-g/incubator-airflow,artwr/airflow,bolkedebruin/airflow,wileeam/airflow,wooga/airflow,r39132/airflow,danielvdende/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,r39132/airflow,sid88in/incubator-airflow,akosel/incubator-airflow,nathanielvarona/airflow