doc: fix formatting in effective go code snippet

Fixes #5403.

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/9100046

doc/effective_go.html

@@ -1805,7 +1805,7 @@ is different from our custom <code>Append</code> function above.
 Schematically, it's like this:
 </p>
 <pre>
-func append(slice []<i>T</i>, elements...<i>T</i>) []<i>T</i>
+func append(slice []<i>T</i>, elements ...<i>T</i>) []<i>T</i>
 </pre>
 <p>
 where <i>T</i> is a placeholder for any given type.  You can't