plugin/header: run go gen and small doc updates (#4754)

* plugin/header: run go gen and small doc updates

Run go gen to enable the new plugin, this should fix the CI failures.
Small tweaks to the docs.

Signed-off-by: Miek Gieben <miek@miek.nl>

* another typo

Signed-off-by: Miek Gieben <miek@miek.nl>

* gofmt

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/header/setup.go

@@ -3,9 +3,9 @@ package header
 import (
 	"fmt"
 
+	"github.com/coredns/caddy"
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
-	"github.com/coredns/caddy"
 )
 
 func init() { plugin.Register("header", setup) }