cmd/proxy-to-grafana: fix package doc code snippet

Markdown isn't supported.

Change-Id: I8d9bb92260c164dc277afbce624f64fc2faf5125
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/proxy-to-grafana/proxy-to-grafana.go

@@ -14,7 +14,6 @@
 //
 // Use this Grafana configuration to enable the auth proxy:
 //
-// ```
 //     [auth.proxy]
 //     enabled = true
 //     header_name = X-WEBAUTH-USER
@@ -23,7 +22,6 @@
 //     whitelist = 127.0.0.1
 //     headers = Name:X-WEBAUTH-NAME
 //     enable_login_token = true
-// ```
 package main
 
 import (