Switch from tabs to spaces in postgresql.conf.sample.

Switch from tabs to spaces in postgresql.conf.sample.

This file is written for 8-space tabs, since we expect that most
users who edit their configuration files use 8-space tabs.
However, most of PostgreSQL is written for 4-space tabs, and at
least one popular web interface defaults to 4-space tabs.  Rather
than trying to standardize on a particular tab width for this file,
let's just switch to spaces.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Discussion: https://postgr.es/m/aReNUKdMgKxLqmq7%40nathan