Fix typo in PHPMailer class DKIM_Add function array autoSignHeaders

[ghost]

Typo ('nessage-id') in array autoSignHeaders of PHPMailer class in DKIM_Add function.
rfc6376 defines no such field as 'nessage-id' is it meant to be 'message-id'.

//Always sign these headers without being asked
//Recommended list from https://tools.ietf.org/html/rfc6376#section-5.4.1
$autoSignHeaders = [
    'from',
    'to',
    'cc',
    'date',
    'subject',
    'reply-to',
    /**
     *  what are we upto here: 'nessage-id' or 'message-id'
     *  rfc6376 defines no such field as 'nessage-id'
     */
    'nessage-id',
    'content-type',
    'mime-version',
    'x-mailer',
];

[Synchro]

Yep, it was a typo introduced during recent changes in DKIM handling. Thanks.