cmd_nand: fix help of nand erase subcommand

Since commit 30486322 (nand erase: .spread, .part, .chip subcommands)
the arguments off and size are no longer optional.

Signed-off-by: Daniel Hobi <daniel.hobi@schmid-telecom.ch>
Cc: Scott Wood <scottwood@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>

common/cmd_nand.c

@@ -693,7 +693,7 @@ U_BOOT_CMD(
 	"    write 'size' bytes starting at offset 'off' with yaffs format\n"
 	"    from memory address 'addr', skipping bad blocks.\n"
 #endif
-	"nand erase[.spread] [clean] [off [size]] - erase 'size' bytes "
+	"nand erase[.spread] [clean] off size - erase 'size' bytes "
 	"from offset 'off'\n"
 	"    With '.spread', erase enough for given file size, otherwise,\n"
 	"    'size' includes skipped bad blocks.\n"