DOC: Remove repeated column assignment in visualization doc
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

The visualization doc had an example where a column in a DataFrame is repeatedly assigned.

```python
df["b"] = df["b"] = df["b"] + np.arange(1000)
```

The expression has been simplified.

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