Doc: Correct number of contexts in LM-4.2.1

fredrikolofsson.com musicalfieldsforever.com

reported:

i believe i found a minor issue halfway down on this page... https://lilypond.org/doc/v2.23/Documentation/learning/properties-of-layout-objects

4.2.1 Properties of layout objects, Finding the context

"Slur_engraver is part of seven Voice contexts"

should be...

"Slur_engraver is part of eight Voice contexts"

_f

This merge request corrects this error.