docs: Set sso_region in s3 config example

[pavelzw]

otherwise you will get the following error:

❯ pixi list
Error:
  × dispatch failure
  ├─▶ other
  ├─▶ the credentials provider was not properly configured
  ╰─▶ ProfileFile provider could not be built: profile `conda` was not defined: `sso_region` was missing

[Hofer-Julian]

Thanks!