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>

doc/api/http.md

@@ -907,9 +907,8 @@ added: v0.4.0
 * `name` {String}
 * Returns: {String}
 
-Reads out a header that's already been queued but not sent to the client.  Note
+Reads out a header that's already been queued but not sent to the client.
-that the name is case insensitive.  This can only be called before headers get
+Note that the name is case insensitive.
-implicitly flushed.
 
 Example: