Bug#829673: clarify who's database prompt

[積丹尼 Dan Jacobson]

Package: postgis
Version: 2.2.2+dfsg-3
Severity: wishlist
File: /usr/share/doc/postgis/README.Debian

We read

  To install the PostGIS extension as well as its topology extension
  in a specific database, you need to run the following SQL commands
  from your database prompt:

    CREATE EXTENSION postgis;
    CREATE EXTENSION postgis_topology;        -- optional

Mention that you need to
su - -s /bin/bash postgres
psql some_database

and then enter the commands. Else you will get an error.