cmd:(cosmetic)Mentioned in 'chpart' command HELP text that it is for MTD devices

Modified the help text of 'chpart' command ,mentioning that it is
for MTD devices.

Signed-off-by: Adarsh Babu Kalepalli <opensource.kab@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/mtdparts.c b/cmd/mtdparts.c
index ed373a8..3048700 100644
--- a/cmd/mtdparts.c
+++ b/cmd/mtdparts.c
@@ -2077,9 +2077,9 @@
 /***************************************************/
 U_BOOT_CMD(
 	chpart,	2,	0,	do_chpart,
-	"change active partition",
+	"change active partition of a MTD device",
 	"part-id\n"
-	"    - change active partition (e.g. part-id = nand0,1)"
+	"    - change active partition (e.g. part-id = nand0,1) of a MTD device"
 );
 
 #ifdef CONFIG_SYS_LONGHELP