MAINTAINERS: assign doc/arch/sandbox.rst

Add doc/arch/sandbox.rst to the scope of SANDBOX.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 85babd1..ed5e354 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -959,6 +959,7 @@
 M:	Simon Glass <sjg@chromium.org>
 S:	Maintained
 F:	arch/sandbox/
+F:	doc/arch/sandbox.rst
 
 SH
 M:	Marek Vasut <marek.vasut+renesas@gmail.com>