Added link to demo paper in README.MD
Added link to demo paper in README.MD

Link "A Demonstration of GeoSpark: A Cluster Computing Framework for Processing Big Spatial Data" has invalid URL.
Updated link to http://www.public.asu.edu/~jiayu2/geospark/publication/GeoSpark_DemoPaper.pdf
Sarwat/GeoSpark,zongsizhang/GeoSpark