Use the preferred <sourcecode> type for DNS RRs

See https://www.rfc-editor.org/rpc/wiki/doku.php?id=sourcecode-types.
Meant to do this in f41d972, but
forgot to `git commit -a`.

Author: davidben

draft-ietf-tls-trust-anchor-ids.md

@@ -269,7 +269,7 @@ The wire format of the SvcParamValue is determined by prefixing each trust ancho
 
 For example, if a TLS server has three available certification paths issued by `32473.1`, `32473.2.1`, and `32473.2.2`, respectively, the DNS record in presentation syntax may be:
 
-~~~ dns
+~~~ dns-rr
 example.net.  7200  IN SVCB 3 server.example.net. (
     tls-trust-anchors=32473.1,32473.2.1,32473.2.2 )
 ~~~