doc: README.mx28_common: Add missing entry into Contents

commit 419ea2d84 (mx28: Split the README into a common part and a m28 specific part)
missed an entry in the Contents section that was part of the original file.

Add the missing entry for completeness.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>

doc/README.mx28_common

@@ -19,6 +19,7 @@ Contents
 1) Prerequisites
 2) Compiling U-Boot for a MX28 based board
 3) Installation of U-Boot for a MX28 based board to SD card
+4) Installation of U-Boot into NAND flash
 
 1) Prerequisites
 ----------------
@@ -146,8 +147,8 @@ NOTE: If the user needs to adjust the start sector, the "mxsboot" tool contains
       a "-p" switch for that purpose. The "-p" switch takes the sector number as
       an argument.
 
-4) Installation of U-Boot for NAND flash
------------------------------------------------
+4) Installation of U-Boot into NAND flash
+-----------------------------------------
 
 To boot a MX28 based board from NAND, set the boot mode DIP switches according to i.MX28
 manual chapter 12.2.1 (Table 12-2), PORT=GPMI, NAND 1.8 V.