correction of documentation on 2.4.1 fret diagram markups

[wmilmoe]

NOTE this is what is currently listed in the documentation and it does not work  I am using \version "2.18.2"



<c e g c' e'>1^\markup {
      \fret-diagram "6-x;5-3;4-2;3-o;2-1;1-o;"
    }



add a # before the first " and it works fine:



<c e g c' e'>1^\markup {
      \fret-diagram #"6-x;5-3;4-2;3-o;2-1;1-o;"
    }