http: fix typo in http.md

Author: mikicho

doc/api/http.md

@@ -626,7 +626,7 @@ added: v0.3.6
 -->
 
 Emitted when the request has been sent. More specifically, this event is emitted
-when the last segment of the response headers and body have been handed off to
+when the last segment of the request headers and body have been handed off to
 the operating system for transmission over the network. It does not imply that
 the server has received anything yet.