docs: mtd: move it to the driver-api book

While I was tempted to move it to admin-guide, as some docs
there are more userspace-faced, there are some very technical
discussions about memory error correction code from the Kernel
implementer's PoV. So, let's place it inside the driver-api
book.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/mtd/nand/raw/nand_ecc.c

@@ -11,7 +11,7 @@
  *   Thomas Gleixner (tglx@linutronix.de)
  *
  * Information on how this algorithm works and how it was developed
- * can be found in Documentation/mtd/nand_ecc.rst
+ * can be found in Documentation/driver-api/mtd/nand_ecc.rst
  */
 
 #include <linux/types.h>