updated a link from sidewalk.umiacs.edu to projectsidewalk.io
updated a link from sidewalk.umiacs.edu to projectsidewalk.io

ProjectSidewalk/SidewalkWebpage,ProjectSidewalk/SidewalkWebpage,ProjectSidewalk/SidewalkWebpage,ProjectSidewalk/SidewalkWebpage,ProjectSidewalk/SidewalkWebpage,ProjectSidewalk/SidewalkWebpage