add missing details from 7.2-to-7.3.md

Add missing configure Redis to use sockets details from https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/7.2-to-7.3.md#5-configure-redis-to-use-sockets.

/cc @saily (original commit in https://github.com/gitlabhq/gitlabhq/blob/master/doc/update/7.2-to-7.3.md#5-configure-redis-to-use-sockets)