plugin/template: fix rcode option documentation (#5328)

* docs: fix incorrect default rcode value, and reference source for valid values
Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin/template/README.md

@@ -27,7 +27,8 @@ template CLASS TYPE [ZONE...] {
 * **REGEX** [Go regexp](https://golang.org/pkg/regexp/) that are matched against the incoming question name. Specifying no regex matches everything (default: `.*`). First matching regex wins.
 * `answer|additional|authority` **RR** A [RFC 1035](https://tools.ietf.org/html/rfc1035#section-5) style resource record fragment
   built by a [Go template](https://golang.org/pkg/text/template/) that contains the reply.
-* `rcode` **CODE** A response code (`NXDOMAIN, SERVFAIL, ...`). The default is `SUCCESS`.
+* `rcode` **CODE** A response code (`NXDOMAIN, SERVFAIL, ...`). The default is `NOERROR`. Valid response code values are
+  per the `RcodeToString` map defined by the `miekg/dns` package in `msg.go`.
 * `fallthrough` Continue with the next plugin if the zone matched but no regex matched.
   If specific zones are listed (for example `in-addr.arpa` and `ip6.arpa`), then only queries for
   those zones will be subject to fallthrough.