Fix markup nits.

Doc/lib/libos.tex

@@ -1515,15 +1515,15 @@ calling the Standard C function \cfunction{system()}, and has the
 same limitations.  Changes to \code{posix.environ}, \code{sys.stdin},
 etc.\ are not reflected in the environment of the executed command.
 
-On \UNIX the return value is the exit status of the process encoded in the
+On \UNIX, the return value is the exit status of the process encoded in the
 format specified for \function{wait()}.  Note that \POSIX{} does not
 specify the meaning of the return value of the C \cfunction{system()}
 function, so the return value of the Python function is system-dependent.
 
-On Windows the return value is that returned by the system shell after
+On Windows, the return value is that returned by the system shell after
 running \var{command}, given by the Windows environment variable
-\code{COMSPEC}: on \code{command.com} systems (Windows 95, 98 and ME)
-this is always \code{0}; on \code{cmd.exe} systems (Windows NT, 2000
+\envvar{COMSPEC}: on \program{command.com} systems (Windows 95, 98 and ME)
+this is always \code{0}; on \program{cmd.exe} systems (Windows NT, 2000
 and XP) this is the exit status of the command run; on systems using
 a non-native shell, consult your shell documentation.