Update render_vis to render.render_vis
Update render_vis to render.render_vis

Comparing the README text here with the code in the [Lucid Tutorial](https://colab.sandbox.google.com/github/tensorflow/lucid/blob/master/notebooks/tutorial.ipynb), looks like the README sample code might be out of date? 

Arguably you could import lucid.optvis.render as itself or give it any other name. But, the example code also uses objectives.channel, so might as well use render.render_vis to make the examples here consistent with the colab tutorial and with how objectives is being used.
tensorflow/lucid,tensorflow/lucid,tensorflow/lucid,tensorflow/lucid