http: fix documentation typo

(Variable is referred to alternately as 'r' and 'req')

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4648042

src/pkg/http/server.go

@@ -570,7 +570,7 @@ func (w *response) Hijack() (rwc net.Conn, buf *bufio.ReadWriter, err os.Error)
 // Handler object that calls f.
 type HandlerFunc func(ResponseWriter, *Request)
 
-// ServeHTTP calls f(w, req).
+// ServeHTTP calls f(w, r).
 func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request) {
 	f(w, r)
 }