Physics-References.md

-* Hillas thinning, Hillias splitting (original reference): [_JK_Hillas-Symposium.pdf](uploads/a178f66e1b5c54b1e3d23b6866f63ebd/_JK_Hillas-Symposium.pdf)
+* Hillas thinning (original reference): ICRC 1981 [_JK_Hillas-Symposium.pdf](uploads/a178f66e1b5c54b1e3d23b6866f63ebd/_JK_Hillas-Symposium.pdf)