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/menu.h

@@ -64,7 +64,7 @@ typedef struct WMenu {
 	int frame_x, frame_y;	       /* position of the frame in root*/
 
 	WMenuEntry **entries;	       /* array of entries. This is shared
-					* by the menu and it's "brother" */
+					* by the menu and its "brother" */
 	short alloced_entries;	       /* number of entries allocated in
 					* entry array */
 	struct WMenu **cascades;	       /* array of cascades */