doc/effective_go.html: fixed the Request channel parameter

R=golang-dev, r
CC=golang-dev, gri
https://golang.org/cl/6010051

doc/effective_go.html

@@ -2479,7 +2479,7 @@ func handle(queue chan *Request) {
     }
 }
 
-func Serve(clientRequests chan *clientRequests, quit chan bool) {
+func Serve(clientRequests chan *Request, quit chan bool) {
     // Start handlers
     for i := 0; i < MaxOutstanding; i++ {
         go handle(clientRequests)