Update the GitHub action for the renamed branch: master → main

Author: bwaldvogel

README.md

@@ -1,6 +1,6 @@
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+[![codecov](https://codecov.io/gh/bwaldvogel/liblinear-java/branch/main/graph/badge.svg?token=Scy4yJzBf7)](https://codecov.io/gh/bwaldvogel/liblinear-java)
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