Fix spelling error in debian/copyright: exeption -> exception

The spelling error is in the quote from the original mail to the
upstream author, so it is borderline to be fixed to silence lintian.

debian/copyright

@@ -9,7 +9,7 @@ License: GPL-2 with OpenSSL exception
 Comment:
  From: =?ISO-8859-1?Q?Jos=E9_Fonseca?= <jrfonseca@users.sourceforge.net>
  To: Salvatore Bonaccorso <salvatore.bonaccorso@gmail.com>
- Subject: Re: OpenSSL exeption for esmtp?
+ Subject: Re: OpenSSL exception for esmtp?
  Date: Wed, 18 Apr 2012 17:09:30 +0100
  Message-ID: <CAH_+9jkK82Av8UPQ7x4Uy1kNJKEV8Uh9as-e5_pqfkBjjMMriQ@mail.gmail.com>
  .