board/loongarch64-efi: fix typo in grub config

Signed-off-by: Scott Fan <fancp2007@gmail.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/loongarch64-efi/grub.cfg

@@ -1,4 +1,4 @@
-/set default="0"
+set default="0"
 set timeout="5"
 
 menuentry "Buildroot" {