so, the steps are probably the "right" steps, but between the book and the film, the "wrong" thing happened there
From what I can gather (though still not sure)...
These 'Nancy's steps' were where her movie death was filmed (the 1968 musical film). But whoever created Nancy's plaque got it muddled. (Unless the plaque was made/put up BEFORE the movie was filmed...in which case, I am baffled!.....(another scenario
The plaque may have been there way before the film was made, but the film took inspiration from the title/meaning of the steps so they acted Nancy's death there as a sort of nod to it? Oh dear, who knows!)