update 404'd source.android.com URLs

Change-Id: Ia902f1bd737eb1ecc0a015e9d63fd8a05cf3ae26
diff --git a/docs/howto_build_SDK.txt b/docs/howto_build_SDK.txt
index e09440b..b9e908c 100644
--- a/docs/howto_build_SDK.txt
+++ b/docs/howto_build_SDK.txt
@@ -68,7 +68,7 @@
 First, setup your development environment and get the Android source code from
 git as explained here:
 
-  http://source.android.com/download
+  http://source.android.com/source/download.html
 
 For example for the cupcake branch: