Documentation/: it's -> its where appropriate

Fix obvious cases of "it's" being used when "its" was meant.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

Documentation/md.txt

@@ -136,7 +136,7 @@ raid_disks != 0.
 
 Then uninitialized devices can be added with ADD_NEW_DISK.  The
 structure passed to ADD_NEW_DISK must specify the state of the device
-and it's role in the array.
+and its role in the array.
 
 Once started with RUN_ARRAY, uninitialized spares can be added with
 HOT_ADD_DISK.