doc/go1.4.html: fix typo

LGTM=adg
R=r, adg
CC=golang-codereviews
https://golang.org/cl/165890043

doc/go1.4.html

@@ -621,7 +621,7 @@ for the decompressors, allowing them to reuse buffers and improve performance.
 
 <li>
 The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
-now supports APLN as defined in <a href="http://tools.ietf.org/html/rfc7301">RFC 7301</a>.
+now supports ALPN as defined in <a href="http://tools.ietf.org/html/rfc7301">RFC 7301</a>.
 </li>
 
 <li>