image: Fix rootfs.mk syntax error

Change-Id: Ie993e49a0db996b72767850a9777465ea6312f57
diff --git a/rootfs.mk b/rootfs.mk
index 1dd169a..0edcb56 100644
--- a/rootfs.mk
+++ b/rootfs.mk
@@ -111,7 +111,7 @@
 	sudo rmdir $(ROOTFS_DIR)
 	sudo sync $(ROOTFS_RAW_IMG)
 	sudo chown ${USER} $(ROOTFS_RAW_IMG)
-	cd $(dir $(ROOTFS_RAW_IMG); sha256sum $(notdir $(ROOTFS_RAW_IMG)) > $(ROOTFS_RAW_IMG).sha256sum
+	cd $(dir $(ROOTFS_RAW_IMG)); sha256sum $(notdir $(ROOTFS_RAW_IMG)) > $(ROOTFS_RAW_IMG).sha256sum
 	$(LOG) rootfs raw-build finished
 endif