DOC: Fix docstrings error SS03 - Summary does not end with a period

[YuechengWu]

when checking for errors with ./scripts/validate_docstrings.py --errors=SS03 --format=azure --prefix=pandas.Series. We get all SS03 errors in pandas/core/generic.py file.

I will be working on this and create a pull request afterwards. Thanks.

[YuechengWu]

Not only in generic.py, some other files too, I will be fixing them all together.