sandbox: Fix README to indicate that vendor name is unset

This brings in a additional small fix which was missed in a recent update
to the README.

Suggested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>

board/sandbox/README.sandbox

@@ -18,8 +18,8 @@ create unit tests which we can run to test this upper level code.
 
 CONFIG_SANDBOX is defined when building a native board.
 
-The chosen vendor and board names are also 'sandbox', so there is a single
-board in board/sandbox.
+The board name is 'sandbox' but the vendor name is unset, so there is a
+single board in board/sandbox.
 
 CONFIG_SANDBOX_BIG_ENDIAN should be defined when running on big-endian
 machines.