use taskapp-demo path when referenced in readme

[mdobrzanski]

The Task App documentation references the app repository as https://github.com/SlashDB/slashdb-react-sdk.git which is a bit misleading. It would be better to use https://github.com/SlashDB/taskapp-demo.git

[Alex-Kamenev]

All occurrences of slashdb-react-sdk have been replaced with taskapp-demo in links on the README.