SF bug 1415455, fix typo in module name

Doc/lib/libsubprocess.tex

@@ -387,7 +387,7 @@ p = Popen(["mycmd", "myarg"], bufsize=bufsize,
 (child_stdout, child_stdin) = (p.stdout, p.stdin)
 \end{verbatim}
 
-The popen2.Popen3 and popen3.Popen4 basically works as subprocess.Popen,
+The popen2.Popen3 and popen2.Popen4 basically works as subprocess.Popen,
 except that:
 
 \begin{itemize}