Fix codecov badge (#80)

Author: Max Jones

README.rst

@@ -14,7 +14,7 @@ such as Keras_. View the |docs| for more info.
 .. |Build Status| image:: https://github.com/pangeo-data/xbatcher/workflows/CI/badge.svg
    :target: https://github.com/pangeo-data/xbatcher/actions
    :alt: github actions build status
-.. |codecov| image:: https://codecov.io/gh/pangeo-data/xbatcher/branch/master/graph/badge.svg
+.. |codecov| image:: https://codecov.io/gh/pangeo-data/xbatcher/branch/main/graph/badge.svg
    :target: https://codecov.io/gh/pangeo-data/xbatcher
    :alt: code coverage
 .. |docs| image:: http://readthedocs.org/projects/xbatcher/badge/?version=latest