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".

doc/WPrefs.1

@@ -7,7 +7,7 @@ WPrefs \- Window Maker configuration tool
 .SH "DESCRIPTION"
 WPrefs.app is the preferences "editor" for the WindowMaker window manager. It
 can be used to set most of the preference options of WindowMaker and define
-it's applications menu. It also can change some settings that do not belong to
+its applications menu. It also can change some settings that do not belong to
 WindowMaker.
 .PP
 Although WPrefs.app is designed to be easy to use, you should read the