[docs] Use 127.0.0.1 in db_config.json instead of localhost

Summary:
For some reason, after coming back from Burning Man, I am unable to connect to my local MariaDB from Node.js with localhost. Once I switched to 127.0.0.1, everything works fine.

I'm not sure if this is due to some macOS update or something else, but I figure this docs change will help other devs avoid this issue.

Test Plan: I confirmed 127.0.0.1 worked for me when localhost didn't

Reviewers: atul, varun, tomek, O1 Nix

Reviewed By: atul

Differential Revision: https://phab.comm.dev/D9187