Update plugin.md
Fix a small error
@@ -8,7 +8,7 @@ The main method that gets called is `ServeDNS`. It has three parameters:
* `dns.ResponseWriter` that is, basically, the client's connection;
* `*dns.Msg` the request from the client.
-`ServeDNS` returns two values, a response code and an error. If the error is not nil CoreDNS,
+`ServeDNS` returns two values, a response code and an error. If the error is not nil, CoreDNS
will return a SERVFAIL to the client. The response code tells CoreDNS if a *reply has been
written by the plugin chain or not*. In the latter case CoreDNS will take care of that.
