Documentation: filesystems: correct possessive "its"

Change occurrences of "it's" that are possessive to "its"
so that they don't read as "it is".

For f2fs.rst, reword one description for better clarity.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-f2fs-devel@lists.sourceforge.net
Cc: linux-xfs@vger.kernel.org
Cc: Christian Brauner <brauner@kernel.org>
Cc: Seth Forshee <sforshee@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: "Christian Brauner (Microsoft)" <brauner@kernel.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Jaegeuk Kim <jaegeuk@kernel.org>
Link: https://lore.kernel.org/r/20220901002828.25102-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>