[AIRFLOW-XXX] Add template_ext to custom operator example (#6787)
[AIRFLOW-XXX] Add template_ext to custom operator example (#6787)

Update Custom Operator -> Templating section, add template_ext field to HelloOperator example.
wooga/airflow,spektom/incubator-airflow,mistercrunch/airflow,spektom/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,mtagle/airflow,spektom/incubator-airflow,DinoCow/airflow,DinoCow/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,apache/airflow,wileeam/airflow,sekikn/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,nathanielvarona/airflow,apache/incubator-airflow,mtagle/airflow,DinoCow/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,apache/airflow,bolkedebruin/airflow,nathanielvarona/airflow,wileeam/airflow,dhuang/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,sekikn/incubator-airflow,mrkm4ntr/incubator-airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,wooga/airflow,lyft/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,DinoCow/airflow,mrkm4ntr/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,wooga/airflow,airbnb/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,spektom/incubator-airflow,airbnb/airflow,danielvdende/incubator-airflow,apache/incubator-airflow,airbnb/airflow,apache/airflow,sekikn/incubator-airflow,wooga/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow,apache/airflow,mistercrunch/airflow,wileeam/airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,apache/airflow,cfei18/incubator-airflow,apache/incubator-airflow,dhuang/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,wileeam/airflow,mtagle/airflow