doc bug

[John Williams]

lilypond-book --filter example is wrong on
<http://www.lilypond.org/doc/v2.5/Documentation/user/out-www/lilypond/Invoking-lilypond_002dbook.html#Invoking-lilypond_002dbook>

Instead of

     lilypond-book --filter='convert-ly --from=2.0.0' my-book.tely

It should have a "-" to read from stdin:

     lilypond-book --filter='convert-ly --from=2.0.0 -' my-book.tely


It might also be nice to tell the user that lilypond book will not
--filter and --process at the same time.  I assumed that --filter would
just do that as an extra step before processing the file normally.