Update dns help with a correct domain name

Closes https://github.com/ipfs/faq/issues/161

License: MIT
Signed-off-by: Kevin Simper <kevin.simper@gmail.com>

core/commands/dns.go

@@ -26,7 +26,7 @@ This command resolves those links to the referenced object.
 
 For example, with this DNS TXT record:
 
-	> dig +short TXT ipfs.io
+	> dig +short TXT _dnslink.ipfs.io
 	dnslink=/ipfs/QmRzTuh2Lpuz7Gr39stNr6mTFdqAghsZec1JoUnfySUzcy
 
 The resolver will give: