doc: imx: psb: Fix missing setexpr arguments

Due to copy-paste error, two of the setexpr arguments were missing.
Add the missing arguments.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Cc: uboot-imx <uboot-imx@nxp.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

doc/imx/misc/psb.rst

@@ -150,7 +150,7 @@ Examples of writing SIT and two copies of bootloader to SD or eMMC:
     => mmc write ${loadaddr} 0x41 0x1
 
     => dhcp ${loadaddr} flash.bin
-    => setexpr blkcnt ${filesize} + 0x1ff && setexpr blkcnt
+    => setexpr blkcnt ${filesize} + 0x1ff && setexpr blkcnt ${blkcnt} / 0x200
     => mmc dev 1
     => mmc write ${loadaddr} 0x42   ${blkcnt}
     => mmc write ${loadaddr} 0x1042 ${blkcnt}