[NTG-context] Odd behaviour with Plex typescript

[Max Chernoff]

> just preload it, as in:
> 
> \usebodyfont[modern]
> 
> \setupbodyfont[plex]
> 
> \setuphead
>    [section]
>    [style={\switchtobodyfont[modern]}]
> 
> \starttext
>      \section{A}
>      A
> \stoptext

That seems to have fixed it. Thanks!

-- Max