Make 'Usage' section in README consistent with current Basic Example at current documentation

[fomightez]

The current Basic Example in the documentation here uses:

%matplotlib ipympl

This is nice/explicit and emphasizes you need ipympl installed. The documentation there further reads, 'Alternatively you can use %matplotlib widget which will have the same effect.' This is good because lets legacy users know they can still use %matplotlib widget.

However, the current README at the ipympl Github repo suggest usage here in 'Usage' section only as %matplotlib widget.
It would be nice if it was consistent and explicit as I see users posting as if %matplotlib ipympl and %matplotlib widget are distinct, despite modern Jupyter needing ipympl.

[github-actions]

👈 Launch a binder notebook on branch fomightez/ipympl/fix_usage_in_README