Fix import path github.com/miekg/coredns -> github.com/coredns/coredns (#547)

This fix fixes import path from
`github.com/miekg/coredns`
->
`github.com/coredns/coredns`

directives_generate.go

@@ -112,7 +112,7 @@ func fatalIfErr(err error) {
 }
 
 const (
-	middlewarePath = "github.com/miekg/coredns/middleware/"
+	middlewarePath = "github.com/coredns/coredns/middleware/"
 	middlewareFile = "middleware.cfg"
 	header         = "// generated by directives_generate.go; DO NOT EDIT\n"
 )