f2fs: fix wrong description in document

As reported in bugzilla, default value of DEF_RAM_THRESHOLD was fixed by
commit 29710bcf ("f2fs: fix wrong percentage"), however leaving wrong
description in document, fix it.

https://bugzilla.kernel.org/show_bug.cgi?id=205203Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.txt

@@ -346,7 +346,7 @@ Files in /sys/fs/f2fs/<devname>
 
  ram_thresh                   This parameter controls the memory footprint used
 			      by free nids and cached nat entries. By default,
-			      10 is set, which indicates 10 MB / 1 GB RAM.
+			      1 is set, which indicates 10 MB / 1 GB RAM.
 
  ra_nid_pages		      When building free nids, F2FS reads NAT blocks
 			      ahead for speed up. Default is 0.