README: correct path to sandbox.rst

sandbox.rst was moved.

Fixes: 2851cc94f301 ("dm: Add documentation for host command and implementation")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 40619ee..24e91b9 100644
--- a/README
+++ b/README
@@ -189,7 +189,7 @@
 specific to be undertaken on a native platform. The sandbox is also used to
 run some of U-Boot's tests.
 
-See doc/arch/sandbox.rst for more details.
+See doc/arch/sandbox/sandbox.rst for more details.
 
 
 Board Initialisation Flow: