fix: missing f-string in --help message

[cjolowicz]

Quick fix for the missing f-string in the --help message for -db and -fb:

  -db {conda,mamba,virtualenv,venv,uv,none}, --default-venv-backend {conda,mamba,virtualenv,venv,uv,none}
                        Virtual environment backend to use by default for Nox sessions, this is ``'virtualenv'`` by default but any of ``{list(ALL_VENVS)!r}`` are accepted.
  -fb {conda,mamba,virtualenv,venv,uv,none}, --force-venv-backend {conda,mamba,virtualenv,venv,uv,none}
                        Virtual environment backend to force-use for all Nox sessions in this run, overriding any other venv backend declared in the Noxfile and ignoring the default backend. Any of ``{list(ALL_VENVS)!r}`` are accepted.

In the long run, it would be nice to clean the --help message up some more. Like those quotes with double backtick and single quote, and the repetition of valid choices.