Documentation: README disagrees with https://matplotlib.org/ipympl/

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## Describe the issue
The [current README](https://github.com/matplotlib/ipympl/blob/3e0a520087d317c7aa7aca38ca1c5c04e7b31f44/README.md) specifies that to use `ipympl` you simply use the following magic:

```
%matplotlib widget
```

However, the [documentation](https://matplotlib.org/ipympl/) specifies the following magic:

```
%matplotlib ipympl
```
The latter works for my system, while the former does not.

## Versions
```
3.10.3 (main, Mar 18 2022, 08:05:10) [GCC 9.4.0]
ipympl version: 0.8.8
Selected Jupyter core packages...
IPython : 8.2.0
ipykernel : 6.12.1
ipywidgets : 7.7.0
jupyter_client : 7.2.1
jupyter_core : 4.9.2
jupyter_server : 1.16.0
jupyterlab : 3.3.2
nbclient : 0.5.13
nbconvert : 6.4.5
nbformat : 5.3.0
notebook : 6.4.10
qtconsole : not installed
traitlets : 5.1.1
Known nbextensions:
config dir: /home/bthayer/.cache/pypoetry/virtualenvs/opf-7uscUANZ-py3.10/etc/jupyter/nbconfig
notebook section
jupyter-matplotlib/extension enabled
- Validating: OK
jupyter-js-widgets/extension enabled
- Validating: OK
JupyterLab v3.3.2
/home/bthayer/.cache/pypoetry/virtualenvs/opf-7uscUANZ-py3.10/share/jupyter/labextensions
jupyter-matplotlib v0.10.5 enabled OK
@jupyter-widgets/jupyterlab-manager v3.1.0 enabled OK (python, jupyterlab_widgets)

```

Compare README.md with the current ipympl documentation at https://matplotlib.org/ipympl/ and verify both magic commands in the reported Jupyter setup. Update the outdated example so the README and documentation agree, then confirm that the usage shown matches the supported command.

Written by the indexing model from the issue text.