test/dm: fdtdec: Corect a typo in dm_test_fdtdec_set_carveout()

It should be "writable".

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/test/dm/fdtdec.c b/test/dm/fdtdec.c
index c2f7b94..999d712 100644
--- a/test/dm/fdtdec.c
+++ b/test/dm/fdtdec.c
@@ -22,7 +22,7 @@
 	blob = malloc(blob_sz);
 	ut_assertnonnull(blob);
 
-	/* Make a writtable copy of the fdt blob */
+	/* Make a writable copy of the fdt blob */
 	ut_assertok(fdt_open_into(gd->fdt_blob, blob, blob_sz));
 
 	resv.start = 0x1000;