minor change to documentation on appending css

[jwhendy]

I just referred to this page on external resources. I think this is a copy/paste error:

You can embed this stylesheet with this URL https://codepen.io/chriddyp/pen/bWLwgP.css.
app.scripts.append_css({'external_url': 'https://codepen.io/chriddyp/pen/bWLwgP.css'})

I believe that should be:

app.css.append_css({'external_url': 'https://codepen.io/chriddyp/pen/bWLwgP.css'})

Sorry if this isn't the right place to post for documentation catches on the actual plot.ly site.

[chriddyp]

Thanks for reporting @jwhendy ! Just added this fix in https://github.com/plotly/dash-docs/commit/03c58ceaea700012485ede6aa58325cbf12f596a