Documentation: mtd: improve nand_ecc.txt for readability and correctness

This patch correct some representation errors, add a little
clarification in some places, and fix indentation problems
for pseudo code.

It also delete one more white space for one place.

Signed-off-by: Wang YanQing <udknight@gmail.com>
[Brian: a few tweaks]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>