fix syntax for haproxy X-Request-Start configuration

source: https://stackoverflow.com/questions/31036893/how-do-i-add-x-request-start-in-haproxy/50060498#50060498

docs/deployment.md

@@ -66,7 +66,8 @@ thread to become available.
 
 * Have your upstream proxy set a header with the time it received the request:
     * nginx: `proxy_set_header X-Request-Start "${msec}";`
-    * haproxy: `http-request set-header X-Request-Start "%t";`
+    * haproxy >= 1.9: `http-request set-header X-Request-Start t=%[date()]%[date_us()]`
+    * haproxy < 1.9: `http-request set-header X-Request-Start t=%[date()]`
 * In your Rack middleware, determine the amount of time elapsed since `X-Request-Start`.
 * To improve accuracy, you will want to subtract time spent waiting for slow clients:
     * `env['puma.request_body_wait']` contains the number of milliseconds Puma spent