[dev-context] typo in core-rul documentation

[Aditya Mahajan]

line 295 says

%D  \startbuffer[tmp-2]
%D  \framed[backgroundachtergrond=fancy]{How Fancy!}
%D  \framed[backgroundachtergrond=fancy,frame=off]{Even More Fancy!}
%D  \stopbuffer

I think that these sould be background rather than 
backgroundachterground

Aditya