reference-point example needs updating

[Mark Polesky]

Carl (et al.),

I think your commit a3b2f9e changed the appearance of the
image in NR 4.4.1 that demonstrates "reference points":

http://lilypond.org/doc/v2.13/Documentation/notation/flexible-vertical-spacing-within-systems#within_002dsystem-spacing-properties

The reference points for ChordNames and NoteNames used to be
the baseline.  If this has changed, the source code for the
image (and its accompanying documentation) needs to be
updated, I think.  I don't have time to do it myself, but I
thought I'd point it out.

Thanks,
Mark