DOC: Typo in `fillna()` documentation
### 

- [X] I have checked that the issue still exists on the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/)


### Location of the documentation

https://pandas.pydata.org/docs/reference/api/pandas.DataFrame.fillna.html

### Documentation problem

Typo in one of the examples for `fillna()`. Specifically, the last example with `df2 = pd.DataFrame(np.zeros((4, 4)), columns=list("ABCE"))` where the last column should be `E` but is instead `D` in the printout.

### Suggested fix for documentation

Either fix to `df2 = pd.DataFrame(np.zeros((4, 4)), columns=list("ABCD"))` or fix the printout.