Doc: Extending: "sole ‘real’ argument" should be simply "argument"

[Mark D. Blackwell]

The Extending LilyPond manual, in section 1.3.4:

http://www.lilypond.org/doc/v2.19/Documentation/extending/adding-articulation-to-notes-_0028example_0029

currently states:

"Now we transform the add-accent function into a music function (a
matter of some syntactic sugar and a declaration of the type of its
sole ‘real’ argument).

    addAccent = #(define-music-function (note-event)
                                         (ly:music?)"

Now, the two words, "sole ‘real’" are mysterious and confusing, since
"note-event" is the only argument.

Formerly, those words referred to the fact that (in version 2.18) the
supplied arguments "parser" and "location" conveyed no information.
Compare version 2.18:

"Now we transform the add-accent function into a music function (a
matter of some syntactic sugar and a declaration of the type of its
sole ‘real’ argument).

    addAccent = #(define-music-function (parser location note-event)
                                         (ly:music?)"

So the present text should be changed to:

"Now we transform the add-accent function into a music function (a
matter of some syntactic sugar and a declaration of the type of its
argument).

    addAccent = #(define-music-function (note-event)
                                         (ly:music?)"