(fix pull request, "utf8" to "utf-8".)
I did the following test.
- compile vuls with this fix.
- I executed the following command and confirmed that garbled display is not done.
+ vuls scan -lang=en -report-mail -cve-dictionary...
+ vuls scan -lang=ja -report-mail -cve-dictionary...
Mail header is as follows.
Message-Id: <...>
Subject: <...>
Content-Type: text/plain; charset=utf8
From: <...>
To: <...>
Cc: <...>
