Fix typo reported by Gerry Weiner <gerry@ucar.edu>.

Doc/lib/libposix.tex

@@ -53,8 +53,7 @@ export statements to the command string for \function{system()} or
 \emph{However:} If you are using this module via the \module{os}
 module (as you should -- see the introduction above), \code{environ}
 is a a mapping object that behaves almost like a dictionary but
-invokes \function{putenv()} automatically called whenever an item is
-changed.
+invokes \function{putenv()} automatically whenever an item is changed.
 \end{datadesc}
 
 \begin{excdesc}{error}