doc: sandbox: Update instructions on quitting

The 'reset' command now resets sandbox but does not quit it. Fix the
instructions.

Fixes: 329dccc0675 ("sandbox: implement reset")
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/arch/sandbox.rst b/doc/arch/sandbox.rst
index 1638b05..8b18a22 100644
--- a/doc/arch/sandbox.rst
+++ b/doc/arch/sandbox.rst
@@ -82,7 +82,7 @@
 You can issue commands as your would normally. If the command you want is
 not supported you can add it to include/configs/sandbox.h.
 
-To exit, type 'reset' or press Ctrl-C.
+To exit, type 'poweroff' or press Ctrl-C.
 
 
 Console / LCD support