docs/backups.md example fails with Network is unreachable error

[geeklingo]

Improve documentation

Link

docs/backups.md

Describe the problem

The backups document details the code to use in GitHub actions to make a backup of the schema and data. Running that code fails with the following error:

Status: Downloaded newer image for ghcr.io/supabase/postgres:17.6.1.002
pg_dumpall: error: connection to server at "db.mydbref.supabase.co" (ipv6_address:3867), port 5432 failed: Network is unreachable
	Is the server running on that host and accepting TCP/IP connections?
error running container: exit 1

Describe the improvement

Updated section with working example