Fix GitHub URL in postgres README (remove www prefix)

Co-authored-by: cschleiden <[email protected]>

Author: Copilot

backend/postgres/README.md

@@ -2,7 +2,7 @@
 
 ## Adding a migration
 
-1. Install [golang-migrate/migrate](https://www.github.com/golang-migrate/migrate)
+1. Install [golang-migrate/migrate](https://github.com/golang-migrate/migrate)
 1. ```bash
    migrate create -ext sql -dir ./db/migrations -seq <name>
    ```