doc: path to u-boot-spl.bin on SiFive Unmatched board

u-boot-spl.bin is built in spl/.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/doc/board/sifive/unmatched.rst b/doc/board/sifive/unmatched.rst
index 24a8899..a994422 100644
--- a/doc/board/sifive/unmatched.rst
+++ b/doc/board/sifive/unmatched.rst
@@ -560,7 +560,7 @@
 
 .. code-block:: none
 
-	dd if=u-boot-spl.bin of=/dev/mtdblock0 bs=4096 seek=5 conv=sync
+	dd if=spl/u-boot-spl.bin of=/dev/mtdblock0 bs=4096 seek=5 conv=sync
 	dd if=u-boot.itb  of=/dev/mtdblock0 bs=4096 seek=261 conv=sync
 
 Power off the board.