Update altair example in docs

[kylebarron]

As of Altair 2.0, running altair.enable_mime_rendering() is not necessary.
https://nteract.gitbooks.io/hydrogen/docs/Usage/Examples.html#static-plots

This works for me:

import altair as alt
from vega_datasets import data

iris = data.iris()

alt.Chart(iris).mark_point().encode(
    x='petalLength',
    y='petalWidth',
    color='species'
)