mtd: concat: Fix a comment referring to an unknown symbol

Fix the comment describing what the mtd_concat_destroy() function
does. It referrers to the concat_mtd_devs symbol which has never
existed (at least not since the beginning of the Git era).
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>

drivers/mtd/mtdconcat.c

@@ -841,10 +841,7 @@ struct mtd_info *mtd_concat_create(struct mtd_info *subdev[],	/* subdevices to c
 	return &concat->mtd;
 }
 
-/*
- * This function destroys an MTD object obtained from concat_mtd_devs()
- */
-
+/* Cleans the context obtained from mtd_concat_create() */
 void mtd_concat_destroy(struct mtd_info *mtd)
 {
 	struct mtd_concat *concat = CONCAT(mtd);