Update readme coverage badge

README.rst

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 .. |ci| image:: https://github.com/alisaifee/flask-limiter/workflows/CI/badge.svg?branch=master
-    :target: https://github.com/alisaifee/flask-limiter/actions?query=branch%3Amaster+workflow%3ACI
-.. |coveralls| image:: https://img.shields.io/coveralls/alisaifee/flask-limiter/master.svg?style=flat-square
-    :target: https://coveralls.io/r/alisaifee/flask-limiter?branch=master
+   :target: https://github.com/alisaifee/flask-limiter/actions?query=branch%3Amaster+workflow%3ACI
+.. |codecov| image:: https://codecov.io/gh/alisaifee/flask-limiter/branch/master/graph/badge.svg
+   :target: https://codecov.io/gh/alisaifee/flask-limiter
 .. |pypi| image:: https://img.shields.io/pypi/v/Flask-Limiter.svg?style=flat-square
-    :target: https://pypi.python.org/pypi/Flask-Limiter
+   :target: https://pypi.python.org/pypi/Flask-Limiter
 .. |license| image:: https://img.shields.io/pypi/l/Flask-Limiter.svg?style=flat-square
-    :target: https://pypi.python.org/pypi/Flask-Limiter
+   :target: https://pypi.python.org/pypi/Flask-Limiter
 .. |docs| image:: https://readthedocs.org/projects/flask-limiter/badge
-    :target: https://flask-limiter.readthedocs.org
+   :target: https://flask-limiter.readthedocs.org
 
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 Flask-Limiter
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-|docs| |ci| |coveralls| |pypi| |license|
+|docs| |ci| |codecov| |pypi| |license|
 
 Flask-Limiter provides rate limiting features to flask routes.
 It has support for a configurable backend for storage