dm: fix alignment of help message of "dm" command

Currently, "help dm" shows as follows:

=> help dm
dm - Driver model low level access

Usage:
dm tree         Dump driver model tree ('*' = activated)
dm uclass        Dump list of instances for each uclass
dm devres        Dump list of device resources for each device

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>

test/dm/cmd_dm.c

@@ -82,7 +82,7 @@ static int do_dm(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 U_BOOT_CMD(
 	dm,	3,	1,	do_dm,
 	"Driver model low level access",
-	"tree         Dump driver model tree ('*' = activated)\n"
+	"tree          Dump driver model tree ('*' = activated)\n"
 	"dm uclass        Dump list of instances for each uclass\n"
 	"dm devres        Dump list of device resources for each device"
 );