Tags: patch

Hello,
I've came across invalid/broken link in Rmail manual which previously
pointed to URL node in GNU Mailutils manual.  I've replaced it with
Mailbox node which is where URLs for different mailboxes are documented
(note that I've also changed printed-manual-title).

I've also capitalized printed-manual-title in the other cross-reference
to mailtuils manual since this is what the correct title should be.

Best

Kuba Ječmínek