Gregorian square neumes ligatures doc.

[Daniel Berjón Díez]

Hi there, I'd just want to report that in this section of the online doc,

5.17.9.2 Gregorian square neumes ligatures

of Lilypond 2.3.16 it appears to be a problem with the fonts
somewhere, the only one that appears to be right is "Porrectus", the
rest consisting of just a stick... very suspicious. ;)

Other than this, congratulations for the tireless effort, it's a
pleasure to watch Lily grow strong and healthy.

Regards,
Daniel

P.S.: The PDF version of the docs has the very same problem