snippet "Accordion register symbols"

[Federico Bruni]

http://lsr.di.unimi.it/LSR/Item?id=929

This snippet contains a typo and a wrong link to NR:

 texidoc = "
Accordion register symbols are available as @code{\\markup} as well as
as standalone music events (as register changes tend to occur between
actual music events. Bass registers are not overly standardized. The
available commands can be found in 'Accordion Registers' in the
Notation Reference.

"

1. typo: there's a missing closing bracket after "music events".

2. There's no 'Accordion Registers' node anymore in the NR:

$ git grep '@node Accordion' Documentation/notation
Documentation/notation/keyboards.itely:586:@node Accordion
Documentation/notation/notation-appendices.itely:1152:@node Accordion 
glyphs

Also, shouldn't we use a cross-ref to link from Snippet to NR manual? 
So it should be:

 available commands can be found in @ruser{Discant symbols}

BTW, even if not used anymore in the documentation, wrong node name is 
also in this obsolete snippet:
http://lsr.di.unimi.it/LSR/Item?id=194

No need to open an issue, I think, since these changes must be made on 
the LSR.

Thanks
Federico