Hey, here's a chance to invent some new terminology!

I would say that Norvig's corrector is a first-order spelling corrector, since it works within the context of a single word.

A second-order corrector would take into account the word before or after it to choose the spelling that is more likely to make sense. ("Their coming" would suggest a correction to "They're coming")

Third-, fourth-, (and so on) order expands the distance of words considered.