DOC: Fix format of basics.rst, following PEP-8 standard
- [x] closes #23792 
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I updated the format of `doc/source/basics.rst` file. 
I do not see any errors when I run `flake8-rst doc/source/basics.rst`.

Please let me know if there are some changes.