btrfs: fix trivial typo for the comment of BTRFS_FREE_INO_OBJECTID

It should be storing, not sotring.
Signed-off-by: NWang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

fs/btrfs/ctree.h

@@ -113,7 +113,7 @@ struct btrfs_ordered_sum;
 #define BTRFS_FREE_SPACE_OBJECTID -11ULL
 
 /*
- * The inode number assigned to the special inode for sotring
+ * The inode number assigned to the special inode for storing
  * free ino cache
  */
 #define BTRFS_FREE_INO_OBJECTID -12ULL