btrfs: fix a comment typo

Just fix a typo in the code comment.

Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Geliang Tang <geliangtang@163.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index abe3a66..7332995 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -1215,7 +1215,7 @@
  * @offset - the offset for the key we'll insert
  *
  * This function writes out a free space cache struct to disk for quick recovery
- * on mount.  This will return 0 if it was successfull in writing the cache out,
+ * on mount.  This will return 0 if it was successful in writing the cache out,
  * or an errno if it was not.
  */
 static int __btrfs_write_out_cache(struct btrfs_root *root, struct inode *inode,