Swap in histogram example

The map_overlay link was broken with the latest update so I replaced it with a histogram example for now.

The landing page gallery on the Bokeh docs has the correct link, although it's the only example that's hosted on the [continuumio](https://github.com/ContinuumIO/bokeh) GitHub Page instead of all the other examples at bokeh.pydata.org. It would be nice to keep the source consistent.