doc: HTTP response getHeader doc fix

According to https://github.com/nodejs/node/issues/10803
getHeader need not be called only before it is flushed implicitly.

PR-URL: https://github.com/nodejs/node/pull/10817


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>