Fix PostgreSQL 15 build instructions

Summary:
This is a follow-up for e79d77e8.

Ref T1888.

Test Plan: salt db-A-001 state.apply roles/dbserver-pgsql/server/build

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1888

Differential Revision: https://devcentral.nasqueron.org/D3287