[Dovecot] Suggested location for single-dbox

I've updated http://wiki2.dovecot.org/MailLocation regarding a typical mail_location entry for dbox.

Previously it said:

 mail_location = dbox:~/dbox

but of course "dbox" is now the old name for "sdbox", so the example should be updated to say "sdbox" and there might/ought also to be an example for "mdbox".

In http://wiki2.dovecot.org/MailboxFormat/dbox the example mail_location for mdbox is given as "mdbox:~/mdbox", so I have followed that for "MailLocation".

So I have changed it to say

 # single-dbox
 mail_location = sdbox:~/dbox

 or:

 # multi-dbox
 mail_location = mdbox:~/mdbox

I didn't know whether to change the directory for sdbox to "~/sdbox" or to leave it as it is.

For the time being I have left it at "~/dbox" --- following the example given should allow new installations of Dovecot 2 to be able to read any existing single-dbox data which used the historically-suggested "~/dbox" location, whilst at the same time avoiding using the old mailbox format name of "dbox".

Bill