Documentation: Fix some grammar mistakes in sync_file.txt

There are two sentences in the Sync File documentation where the
english is a little off. This patch is an attempt to fix these.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>