Replace `iris` with `df`

[chriddyp]

This comes up in all of the workshops that I do - folks don't understand what "iris" is but they do understand the universal "df" variable name.

It would be helpful if we rewrote examples like this:

import plotly.express as px
iris = px.data.iris()
fig = px.scatter(iris, x="sepal_width", y="sepal_length")
fig.show()

to be

import plotly.express as px
df = px.data.iris()
fig = px.scatter(df, x="sepal_width", y="sepal_length")
fig.show()

cc @emmanuelle @nicolaskruchten @Mahdis-z

[nicolaskruchten]

Done and deployed.