DOC: Fixed examples in `pandas/core/accessor.py`
Since this changes require an interactive Ipython shell (I think), we can't run check them from pytest.

This is what the docs looks like:

Before:

![Screenshot_20200403_170159](https://user-images.githubusercontent.com/50263213/78368644-9c0b8400-75cc-11ea-9061-24652f2bdfca.png)

After:
![register_acc](https://user-images.githubusercontent.com/50263213/78368530-71b9c680-75cc-11ea-8ab5-d2ee2eadc727.png)