fix link (#3719)

Signed-off-by: Chris O'Haver <cohaver@infoblox.com>

plugin.md

@@ -31,7 +31,7 @@ code.
 See a couple of blog posts on how to write and add plugin to CoreDNS:
 
 * <https://blog.coredns.io/2017/03/01/how-to-add-plugins-to-coredns/>
-* <https://blog.coredns.io/2016/12/19/writing-plugin-for-coredns/>, slightly older, but useful.
+* <https://blog.coredns.io/2016/12/19/writing-plugins-for-coredns/>, slightly older, but useful.
 
 ## Logging