Update TSIG doc header to avoid godoc oddity
Update TSIG doc header to avoid godoc oddity

For some reason godoc treats it as plain text because it contains
"(TSIG)".
rolandshoemaker/dns,benbearchen/dns,skynet/dns,hugozhu/dns,ezoic/dns,Git-Host/dns,asergeyev/dns,cloudflare/dns,michaelharo/dns,ym/go-dns,FiloSottile/dns,flebarbe/dns,tempbottle/dns,bragr/dns,josephyzhou/dns,ErikDubbelboer/dns,bboreham/dns,jdef/dns,janstk/dns,AlexCheong/dns,Psiphon-Inc/dns,miekg/dns,jmhodges/dns,skyportsystems/dns,curtiszimmerman/dns,tsenart/dns,letsencrypt/dns,modulexcite/dns