[AIRFLOW-XXX] Minor fix of docs/scheduler.rst (#4115)
[AIRFLOW-XXX] Minor fix of docs/scheduler.rst (#4115)

DaskExecutor is not mentioned in docs/scheduler.rst,
while it's listed as one of the main executors in 
`airflow/config_templates/default_airflow.cfg`.
Acehaidrey/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,adamhaney/airflow,DinoCow/airflow,sekikn/incubator-airflow,Acehaidrey/incubator-airflow,wooga/airflow,jgao54/airflow,cfei18/incubator-airflow,mistercrunch/airflow,DinoCow/airflow,jgao54/airflow,wooga/airflow,Fokko/incubator-airflow,mtagle/airflow,mrkm4ntr/incubator-airflow,Fokko/incubator-airflow,jgao54/airflow,apache/airflow,apache/incubator-airflow,mistercrunch/airflow,fenglu-g/incubator-airflow,artwr/airflow,danielvdende/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,r39132/airflow,mistercrunch/airflow,mtagle/airflow,malmiron/incubator-airflow,spektom/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,dhuang/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,DinoCow/airflow,r39132/airflow,cfei18/incubator-airflow,adamhaney/airflow,adamhaney/airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,nathanielvarona/airflow,fenglu-g/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,wileeam/airflow,spektom/incubator-airflow,apache/airflow,apache/airflow,r39132/airflow,airbnb/airflow,airbnb/airflow,mtagle/airflow,wooga/airflow,wooga/airflow,mrkm4ntr/incubator-airflow,r39132/airflow,bolkedebruin/airflow,malmiron/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,adamhaney/airflow,jgao54/airflow,mrkm4ntr/incubator-airflow,apache/airflow,nathanielvarona/airflow,artwr/airflow,fenglu-g/incubator-airflow,sekikn/incubator-airflow,wileeam/airflow,artwr/airflow,nathanielvarona/airflow,wileeam/airflow,malmiron/incubator-airflow,DinoCow/airflow,apache/airflow,Fokko/incubator-airflow,mtagle/airflow,lyft/incubator-airflow,apache/airflow,apache/incubator-airflow,sekikn/incubator-airflow,wileeam/airflow,malmiron/incubator-airflow,mistercrunch/airflow,airbnb/airflow,danielvdende/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,spektom/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,artwr/airflow,lyft/incubator-airflow,fenglu-g/incubator-airflow,spektom/incubator-airflow,Fokko/incubator-airflow