fixing image references in README
fixing image references in README

isisaddons/isis-app-todoapp,zhiqinghuang/isis-app-todoapp,isisaddons/isis-app-todoapp,zhiqinghuang/isis-app-todoapp,isisaddons/isis-app-todoapp,zhiqinghuang/isis-app-todoapp,zhiqinghuang/isis-app-todoapp,isisaddons/isis-app-todoapp