MAINTAINERS: assign doc/arch/sandbox.rst

Add doc/arch/sandbox.rst to the scope of SANDBOX.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: NSimon Glass <sjg@chromium.org>

MAINTAINERS

@@ -959,6 +959,7 @@ SANDBOX
 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>