docs: mtd: Avoid htmldocs warnings

Moving files around produced the following warnings:

       Error: Cannot open file drivers/mtd/nand/raw/nand_ecc.c
       Error: Cannot open file drivers/mtd/nand/raw/nand_ecc.c

Fix one by just dropping the reference because it is not relevant, the
other by using a better noun instead of a file name.
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201113123831.32429-1-miquel.raynal@bootlin.com

Documentation/driver-api/mtd/nand_ecc.rst

@@ -5,7 +5,7 @@ NAND Error-correction Code
 Introduction
 ============
 
-Having looked at the linux mtd/nand driver and more specific at nand_ecc.c
+Having looked at the linux mtd/nand Hamming software ECC engine driver
 I felt there was room for optimisation. I bashed the code for a few hours
 performing tricks like table lookup removing superfluous code etc.
 After that the speed was increased by 35-40%.

Documentation/driver-api/mtdnand.rst

@@ -972,9 +972,6 @@ hints" for an explanation.
 .. kernel-doc:: drivers/mtd/nand/raw/nand_base.c
    :export:
 
-.. kernel-doc:: drivers/mtd/nand/raw/nand_ecc.c
-   :export:
-
 Internal Functions Provided
 ===========================