DOC: remove double curly braces in the DataFrame.add_prefix docstring
- [ x] closes "https://github.com/noatamir/scipy-2023-sprint/issues/10"
- [ 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
- [ x] 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.

@noatamir  and @phofl Scipy2023 sprint
/preview Website preview of this PR available at: https://pandas.pydata.org/preview/54142/ Thanks @vvaidy!