doc: HTTP response getHeader doc fix

According to #10803
getHeader need not be called only before it is flushed implicitly.

PR-URL: #10817
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: fhalde

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
-that the name is case insensitive.  This can only be called before headers get
-implicitly flushed.
+Reads out a header that's already been queued but not sent to the client.
+Note that the name is case insensitive.
 
 Example: