doc/effective_go.html: fix unescaped less-than character

Why doesn't tidy complain about these?
Found by manlio.perillo.
Fixes #6571

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/14608044

doc/effective_go.html

@@ -2953,7 +2953,7 @@ func handle(r *Request) {
 }
 
 func init() {
-    for i := 0; i < MaxOutstanding; i++ {
+    for i := 0; i &lt; MaxOutstanding; i++ {
         sem &lt;- 1
     }
 }