Use pgx5 connection string in README

Author: dhui

database/pgx/v5/README.md

@@ -2,7 +2,7 @@
 
 This package is for [pgx/v5](https://pkg.go.dev/github.com/jackc/pgx/v5). A backend for the older [pgx/v4](https://pkg.go.dev/github.com/jackc/pgx/v4). is [also available](..).
 
-`pgx://user:password@host:port/dbname?query`
+`pgx5://user:password@host:port/dbname?query`
 
 | URL Query  | WithInstance Config | Description |
 |------------|---------------------|-------------|