doc: fix download link on front page

R=golang-dev
CC=golang-dev
https://golang.org/cl/5730053

doc/root.html

@@ -39,7 +39,7 @@ simple, reliable, and efficient software.
 
 <div id="gopher"></div>
 
-<a href="/install/" id="start">
+<a href="/doc/install" id="start">
 <div class="big">Download Go</div>
 <div class="desc">
 Binary distributions available for<br>