plugin/nsid: Small doc cleanup (#1293)

add an example and some typos fixed.

plugin/nsid/README.md

 # nsid
 
-*nsid* add an identifier of this server to each reply.
+*nsid* adds an identifier of this server to each reply.
 
 This plugin implements RFC 5001 and adds an EDNS0 OPT resource record to replies that uniquely
-identifies the server. This can be useful in anycast setups to see which server was responsible for
+identify the server. This is useful in anycast setups to see which server was responsible for
 generating the reply and for debugging.
 
 ## Syntax
@@ -22,6 +22,25 @@ Enable nsid:
 
 ~~~ corefile
 . {
-    nsid
+    whoami
+    nsid Use The Force
 }
 ~~~
+
+And now a client with NSID support will see an OPT record with the NSID option:
+
+~~~ sh
+% dig +nsid @localhost a whoami.example.org
+
+;; Got answer:
+;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 46880
+;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 3
+
+....
+
+; OPT PSEUDOSECTION:
+; EDNS: version: 0, flags:; udp: 4096
+; NSID: 55 73 65 20 54 68 65 20 46 6f 72 63 65 ("Use The Force")
+;; QUESTION SECTION:
+;whoami.example.org.		IN	A
+~~~

plugin/nsid/nsid.go

@@ -29,7 +29,6 @@ func (n Nsid) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (i
 			if _, ok := o.(*dns.EDNS0_NSID); ok {
 				nw := &ResponseWriter{ResponseWriter: w, Data: n.Data}
 				return plugin.NextOrFailure(n.Name(), n.Next, ctx, nw, r)
-
 			}
 		}
 	}