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/switchmenu.c

@@ -190,7 +190,7 @@ void UpdateSwitchMenu(WScreen * scr, WWindow * wwin, int action)
 	 *    1.  When a window is created.
 	 *    2.  When a window is destroyed.
 	 *
-	 *    3.  When a window changes it's title.
+	 *    3.  When a window changes its title.
 	 *    4.  When a window changes its workspace.
 	 */
 	if (action == ACTION_ADD) {