nixos/postgresql: fix documentation markdown

Without the change the doc build fails as:

    $ nix build -f nixos config.system.build.manual.manualHTML -L
    ...
    nixos-manual-html> RuntimeError: heading in line 366 does not have an id

nixos/modules/services/databases/postgresql.md

@@ -363,6 +363,6 @@ postgresql.withJIT.pname
 
 evaluates to `"foobar"`.
 
-## Notable differences to upstream
+## Notable differences to upstream {#module-services-postgres-upstream-deviation}
 
 - To avoid circular dependencies between default and -dev outputs, the output of the `pg_config` system view has been removed.