MAINTAINERS: Fix syntax and update filename for FDT

Let get_maintainers.pl pick up the new cmd/fdt.c.

Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

MAINTAINERS

@@ -272,7 +272,7 @@ F:	lib/fdtdec*
 F:	lib/libfdt/
 F:	include/fdt*
 F:	include/libfdt*
-F.	common/cmd_fdt.c
+F:	cmd/fdt.c
 F:	common/fdt_support.c
 
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