Corrected pd.merge indicator type hint
The argument "indicator" in [merge ](https://pandas.pydata.org/docs/reference/api/pandas.merge.html ) should be "str | bool" instead of just string.

- [ ] closes #xxxx (Replace xxxx with the Github issue number)
- [x] [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).
- [X] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.
Anyone knows why is this check failing? "Ubuntu / actions-310.yaml pyarrow=5 single_cpu (pull_request)" when all others go well? I think the PR is still ok. Thanks @PabloRuizCuevas 