DOC: Fixed the doctsring for _set_axis_name (GH 22895)
Fixed the docstring of the function `_set_axis_name` in `pandas/core/generic.py` (#22895)
Hello @bkjchoi72! Thanks for submitting the PR.

 - In the file [`pandas/core/generic.py`](https://github.com/bkjchoi72/pandas/blob/a43d16f562be824599e04d9e1cbd29315c89c36d/pandas/core/generic.py), following                are the PEP8 issues :
> [Line 1231:80](https://github.com/bkjchoi72/pandas/blob/a43d16f562be824599e04d9e1cbd29315c89c36d/pandas/core/generic.py#L1231): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (98 > 79 characters)
> [Line 1232:80](https://github.com/bkjchoi72/pandas/blob/a43d16f562be824599e04d9e1cbd29315c89c36d/pandas/core/generic.py#L1232): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (92 > 79 characters)
> [Line 1233:80](https://github.com/bkjchoi72/pandas/blob/a43d16f562be824599e04d9e1cbd29315c89c36d/pandas/core/generic.py#L1233): [E501](https://duckduckgo.com/?q=pep8%20E501) line too long (99 > 79 characters)


