doc: Improve {archive,restore}_command for compressed logs

The commands mentioned in the docs with gzip and gunzip did not prefix
the archives with ".gz" and used inconsistent paths for the archives,
which can be confusing.

Reported-by: Philipp Gramzow
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/161397938841.15451.13129264141285167267@wrigleys.postgresql.org

doc/src/sgml/backup.sgml

@@ -1520,11 +1520,11 @@ tar -rf /var/lib/pgsql/backup.tar /var/lib/pgsql/archive/
       If archive storage size is a concern, you can use
       <application>gzip</application> to compress the archive files:
 <programlisting>
-archive_command = 'gzip &lt; %p &gt; /var/lib/pgsql/archive/%f'
+archive_command = 'gzip &lt; %p &gt; /mnt/server/archivedir/%f.gz'
 </programlisting>
       You will then need to use <application>gunzip</application> during recovery:
 <programlisting>
-restore_command = 'gunzip &lt; /mnt/server/archivedir/%f &gt; %p'
+restore_command = 'gunzip &lt; /mnt/server/archivedir/%f.gz &gt; %p'
 </programlisting>
      </para>
     </sect3>