modified the comments of block device geometry structure, to tell everyone the...

modified the comments of block device geometry structure, to tell everyone the unit of block_size is byte.

include/rtdef.h

@@ -844,7 +844,7 @@ struct rt_device_blk_geometry
 {
     rt_uint32_t sector_count;                           /**< count of sectors */
     rt_uint32_t bytes_per_sector;                       /**< number of bytes per sector */
-    rt_uint32_t block_size;                             /**< size to erase one block */
+    rt_uint32_t block_size;                             /**< number of bytes to erase one block */
 };
 
 /**