Remove apostrophes from instances of possessive "its".

I noticed one instance of this while looking at the code the other day,
and after a quick grep, realized it happened a *lot*!  One of the many
frustrating things about the English language is that we use apostrophes
to make pretty much everything possessive *except* the pronoun "it".
In that case, we use "its".  "It's" is reserved for the contraction
meaning "it is" or "it has".

src/screen.h

@@ -57,7 +57,7 @@ typedef struct WDrawerChain {
 } WDrawerChain;
 
 /*
- * each WScreen is saved into a context associated with it's root window
+ * each WScreen is saved into a context associated with its root window
  */
 typedef struct _WScreen {
     int	screen;			       /* screen number */