Chart: Rename `dashboard` to `Webserver` in Installation Notes (#15808)
Chart: Rename `dashboard` to `Webserver` in Installation Notes (#15808)

Before:

```
Your release is named airflow.
You can now access your dashboard(s) by executing the following command(s) and visiting the corresponding port at localhost in your browser:

Airflow dashboard:        kubectl port-forward svc/airflow-webserver 8080:8080 --namespace airflow
Flower dashboard:         kubectl port-forward svc/airflow-flower 5555:5555 --namespace airflow
```

After:
```
Your release is named airflow.
You can now access your service(s) by executing the following command(s) and visiting the corresponding port at localhost in your browser:

Airflow Webserver:        kubectl port-forward svc/airflow-webserver 8080:8080 --namespace airflow
Flower dashboard:         kubectl port-forward svc/airflow-flower 5555:5555 --namespace airflow
```
lyft/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,mistercrunch/airflow,apache/airflow,apache/airflow,Acehaidrey/incubator-airflow,mistercrunch/airflow,dhuang/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,apache/airflow,sekikn/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow,dhuang/incubator-airflow,apache/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,mistercrunch/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,sekikn/incubator-airflow,bolkedebruin/airflow,nathanielvarona/airflow,nathanielvarona/airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,bolkedebruin/airflow