Fix code coverage badge

[PanderMusubi]

Proposal to remove the coverall badge from the readme as the last check is stuck at 92% in 2021. (Or fix the coverall runs.)

Fix the codecov error as seen in https://github.com/helloflask/bootstrap-flask/actions/runs/5927943152/job/16072510875#step:7:44

Add codecov badge to the readme file.

[greyli]

Replace the current badge with:

[![codecov](https://codecov.io/gh/helloflask/bootstrap-flask/graph/badge.svg?token=JdY3VA1z0G)](https://codecov.io/gh/helloflask/bootstrap-flask)