f2fs: fix wrong description of compress feature in f2fs.rst

f2fs will try compressing data in cluster only when "all logical
blocks in cluster contain valid data" rather than "all logical
blocks in file are valid".
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/filesystems/f2fs.rst

@@ -739,8 +739,8 @@ Compression implementation
 
 - In order to eliminate write amplification during overwrite, F2FS only
   support compression on write-once file, data can be compressed only when
-  all logical blocks in file are valid and cluster compress ratio is lower
-  than specified threshold.
+  all logical blocks in cluster contain valid data and compress ratio of
+  cluster data is lower than specified threshold.
 
 - To enable compression on regular inode, there are three ways: