CONTRIBUTING.md: Clarify why the the setup mentions two different version of postgres to be used

[palash25]

I am assuming it is a typo but i just want to confirm this so I am creating an issue instead of a PR

The Apply configurations sections of the contributing guide asks to apply configurations to postgres version 15 <your-editor> ~/.pgrx/data-15/postgresql.conf

but when you run make setup the configurations are applied to version 17 which is hardcoded in the Makefile.

Is this is a mistake? the apply configurations step should either mention 17 as the postgres version or not mention any version at all and leave it up to the user. Thoughts?