example/appengine-hello: Update Go SDK URL

Change-Id: I1029533cdc6a1ebfc546808e16633c19dfae834d
Reviewed-on: https://go-review.googlesource.com/10874
Reviewed-by: Andrew Gerrand <adg@golang.org>

appengine-hello/README.md

diff --git a/appengine-hello/README.md b/appengine-hello/README.md
index a680576..ace365e 100644
--- a/appengine-hello/README.md
+++ b/appengine-hello/README.md
@@ -2,8 +2,8 @@
 Go.
 
 To run the application locally you need the install the [Go Cloud
-SDK](https://cloud.google.com/sdk/) and execute the next command from
-the directory containing this file:
+SDK](https://cloud.google.com/appengine/docs/go/#Go_tools) and execute the next
+command from the directory containing this file:
 
     $ goapp serve app.yaml