Update README to show how to invoke fill-db script
Update README to show how to invoke fill-db script

Before it was `fill_db` but since Flask upgraded to a new version of
click, which replaces the underscore with a dash for command line
invokation.

See: https://github.com/pallets/click/commit/5d1df0e042b2f8b0dee8f6dd9ce74edce331a950

rtzll/flask-todolist,0xfoo/flask-todolist,rtzll/flask-todolist,polyfunc/flask-todolist,0xfoo/flask-todolist,rtzll/flask-todolist,polyfunc/flask-todolist,rtzll/flask-todolist,polyfunc/flask-todolist,0xfoo/flask-todolist