cmd/proxy-to-grafana: fix package doc code snippet
Markdown isn't supported.
//
// Use this Grafana configuration to enable the auth proxy:
//
-// ```
// [auth.proxy]
// enabled = true
// header_name = X-WEBAUTH-USER
// whitelist = 127.0.0.1
// headers = Name:X-WEBAUTH-NAME
// enable_login_token = true
-// ```
package main
import (
