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/rt-mutex-design.txt

@@ -657,7 +657,7 @@ here.
 
 The waiter structure has a "task" field that points to the task that is blocked
 on the mutex.  This field can be NULL the first time it goes through the loop
-or if the task is a pending owner and had it's mutex stolen.  If the "task"
+or if the task is a pending owner and had its mutex stolen.  If the "task"
 field is NULL then we need to set up the accounting for it.
 
 Task blocks on mutex