Add Content-Type header to report/mail.go .

(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: <...>

report/mail.go

@@ -52,6 +52,7 @@ func (w MailWriter) Write(scanResults []models.ScanResult) (err error) {
 		headers["To"] = to
 		headers["Cc"] = cc
 		headers["Subject"] = subject
+		headers["Content-Type"] = "text/plain; charset=utf-8"
 
 		var message string
 		for k, v := range headers {