DOC: Change "Convention for examples" to use Series name as ser instead of s
- [] closes #53271 (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

> thanks @Jakaho ! there's a couple missing: `def head` and `def plot`, but the rest looks good!

Hey @MarcoGorelli I believe I finished up the remaining changes at https://github.com/pandas-dev/pandas/pull/53312. > > thanks @Jakaho ! there's a couple missing: `def head` and `def plot`, but the rest looks good!
> 
> Hey @MarcoGorelli I believe I finished up the remaining changes at #53312.

indeed, let's go with that one - @Jakaho for the future, if someone has already commented that they're working on an issue, please wait at least a week before taking over. Thanks anyway for your PR!