Fix availability declaration for popen2 module; report by Barry Scott

<barry@scottb.demon.co.uk>.

Doc/lib/libpopen2.tex

@@ -2,7 +2,7 @@
          Subprocesses with accessible standard I/O streams}
 
 \declaremodule{standard}{popen2}
-  \platform{Unix}
+  \platform{Unix, Windows}
 \modulesynopsis{Subprocesses with accessible standard I/O streams.}
 \sectionauthor{Drew Csillag}{drew_csillag@geocities.com}