using bfg or filter-repo, I want to replace text in specific file

Question

my git repo has above files with directory,

• service-a
  • app.config
• service-b
  • app.config

I want to replace the text in service-a/app.config, not service-b/app.config using bfg or filter-repo.

but those looks don't support it. How can I do that?

thanks