Fix graph.ts: matrix2 data now matches ascii art

In matrix2 two vertices (ID 2, ID 6) were connected to themselves instead of their neighbors -> classic off by one errors (and I really hope I did not screw up)

Beware: Sorry, I did not make sure that the unit tests still work because I do this course in Odinlang, not TypeScript! If this is a no-go (totally reasonable!), please just close the PR.

(Since there is currently no license in this repo, I'll explicitly state that my shitty little contribution is provided under the same license the Prime might choose later on)