DatePickerRange: Add default value for `minimum_nights` in doc

[EBoisseauSierra]

I have noticed that dcc.DatePickerRange has a default value for minimum_nights set to 1.
(I.e., in a bare component, you must select a end_date strictly later than start_date; yet if you set minimum_nights=0, then you can select the same day for start_date and end_date.)

However, in the documentation, the default value for minimum_nights is not defined (… what led me foolishly look for alternatives involving two DatePickerSingle, but that's another story).

➥ Could we make explicit in the documentation what default value is used for minimum_nights?

---

# import external modules
import dash
import dash_core_components as dcc

# import external functions
from datetime import datetime

app = dash.Dash(
    __name__,
)

app.layout = dcc.DatePickerRange(
    # minimum_nights=0,
    initial_visible_month=datetime(
        datetime.now().year,
        datetime.now().month,
        1
    ),
)

if __name__ == '__main__':
    app.run_server(debug=True)

[Marc-Andre-Rivet]

@EBoisseauSierra Thanks for pointing this out.
Ref: https://github.com/airbnb/react-dates/blob/d572bea9029739089bd200026be3aad7684a8b02/src/components/DateRangePicker.jsx#L108