Update README.md

fix codecov badge

README.md

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 <!-- badges: start -->
 [![Project Status: Concept – Minimal or no implementation has been done yet, or the repository is only intended to be a limited example, demo, or proof-of-concept.](https://www.repostatus.org/badges/latest/concept.svg)](https://www.repostatus.org/#concept)
 [![R-CMD-check.yaml](https://github.com/seroanalytics/serovizr/actions/workflows/R-CMD-check.yaml/badge.svg)](https://github.com/seroanalytics/serovizr/actions/workflows/R-CMD-check.yaml)
-[![Codecov test coverage](https://codecov.io/gh/seroanalytics/serovizr/branch/master/graph/badge.svg)](https://codecov.io/gh/seroanalytics/serovizr?branch=master)
+[![codecov](https://codecov.io/gh/seroanalytics/serovizr/graph/badge.svg?token=oFACWrbYep)](https://codecov.io/gh/seroanalytics/serovizr)
 <!-- badges: end -->
 
 R API for the SeroViz app. Based on the [porcelain](https://github.com/reside-ic/porcelain) framework.