incsp_1: "incidency" -> "incidence"

[Jesse Alama]

The title of INCSP_1 is "Axioms of Incidency".  That should be
"incidence"; "incidency" isn't an English word.  There are a handful of
uses of "incidency" in the article; those should all be changed.

Thanks,

Jesse

PS I'd like to use this material in an upcoming course on non-Euclidean
geometry that I'll be teaching for high school students at Stanford; I'd
love to know if there is any more formal geometry in the MML that I
could talk about.

--
Jesse Alama (alama@...)