doc/effective_go.html: fix syntax error in code example

Fixes #5333.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/8698046

doc/effective_go.html

@@ -2947,7 +2947,7 @@ func Serve(queue chan *Request) {
         go func() {
             process(req)
             sem <- 1
-        }
+        }()
     }
 }</pre>