fix 4 lgtm alerts
Fix name of methods as `rolling_mean` instead of `rolling_median` following a43c1576ce3d94bc82f7cdd63531280ced5a9fa0.

I'm a dev on lgtm.com so I'm very biased but these were flagged [here](https://lgtm.com/projects/g/pydata/pandas/rev/a43c1576ce3d94bc82f7cdd63531280ced5a9fa0) and you can catch such issues (and more subtle ones) before they find their way into master by giving a go at [PR integration](https://lgtm.com/docs/lgtm/config/pr-integration) :)

Hope that helps! 
