doc: escape some HTML in Effective Go code examples

Change-Id: I4204e268c7220a50ceb270432067850ec2b5af80
Reviewed-on: https://go-review.googlesource.com/41230
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/effective_go.html

@@ -1833,7 +1833,7 @@ for a min function that chooses the least of a list of integers:
 </p>
 <pre>
 func Min(a ...int) int {
-    min := int(^uint(0) >> 1)  // largest int
+    min := int(^uint(0) &gt;&gt; 1)  // largest int
     for _, i := range a {
         if i &lt; min {
             min = i