fixing lilypond-book invocation for texinfo files

[Werner LEMBERG]

In lilypond-book.itely, you can find for example this:

  @example
  lilypond-book --process="lilypond --format=tex --tex \
                --header=texidoc test.texinfo
  @end example

This hasn't worked even before my current efforts to remove the `tex'
and `texstr' backends.  I won't touch this part of the documentation
since I don't know how to do that currently.  Please fix.


    Werner