f2fs: fix a spelling error

Delete the letter 'e' before 'number'
Signed-off-by: xuyehan <xuyehan@xiaomi.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -276,7 +276,7 @@ Date		April 2019
 Contact:	"Daniel Rosenberg" <drosen@google.com>
 Description:	If checkpoint=disable, it displays the number of blocks that
 		are unusable.
-		If checkpoint=enable it displays the enumber of blocks that
+		If checkpoint=enable it displays the number of blocks that
 		would be unusable if checkpoint=disable were to be set.
 
 What:		/sys/fs/f2fs/<disk>/encoding