Fixed PEP8 errors in doc/source/whatsnew/v0.15.*
- [x] closes #24239 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
Fixed PEP8 errors in **doc/source/whatsnew/v0.15.***, still can't get rid of **line too long ( x >79)** where x is the number of characters in line.

