doc: fix root.html styles

R=r
CC=golang-dev
https://golang.org/cl/7442045

doc/root.html

@@ -53,11 +53,11 @@ simple, reliable, and efficient software.
 <div id="gopher"></div>
 
 <a href="/doc/install" id="start">
-<div class="big">Download Go</div>
-<div class="desc">
+<span class="big">Download Go</span>
+<span class="desc">
 Binary distributions available for<br>
 Linux, Mac OS X, Windows, and more.
-</div>
+</span>
 </a>
 
 </div>

doc/style.css

@@ -267,10 +267,12 @@ a#start {
 	border-radius: 5px;
 }
 a#start .big {
+	display: block;
 	font-weight: bold;
 	font-size: 20px;
 }
 a#start .desc {
+	display: block;
 	font-size: 14px;
 	font-weight: normal;
 	margin-top: 5px;