Rename lib__future__.tex to libfuture.tex and add link in lib.tex.

Doc/lib/lib.tex

@@ -101,6 +101,7 @@ and how to embed it in other applications.
 \input{libuser}
 \input{libbltin}                % really __builtin__
 \input{libmain}                 % really __main__
+\input{libfuture}               % really __future__
 
 \input{libstrings}              % String Services
 \input{libstring}