fix(dns.go): Pref is now Preference
fix(dns.go): Pref is now Preference

Simple mistake from, what I guess, was an earlier API change.

miekg/dns,ezoic/dns,cloudflare/dns,Psiphon-Inc/dns,flebarbe/dns,letsencrypt/dns,AlexCheong/dns,janstk/dns,bboreham/dns,jmhodges/dns,ym/go-dns,Git-Host/dns,tsenart/dns,skyportsystems/dns,jdef/dns,FiloSottile/dns,ErikDubbelboer/dns,michaelharo/dns,bragr/dns,rolandshoemaker/dns,josephyzhou/dns,skynet/dns,modulexcite/dns,curtiszimmerman/dns,tempbottle/dns,hugozhu/dns,asergeyev/dns,benbearchen/dns