Update coverage badge (#53)

closes #47

README.md

@@ -3,7 +3,7 @@
 A Go library that implements a Sparse Merkle tree for a key-value map. The tree implements the same optimisations specified in the [Libra whitepaper][libra whitepaper], to reduce the number of hash operations required per tree operation to O(k) where k is the number of non-empty elements in the tree.
 
 [![Tests](https://github.com/celestiaorg/smt/actions/workflows/test.yml/badge.svg)](https://github.com/celestiaorg/smt/actions/workflows/test.yml)
-[![Coverage Status](https://coveralls.io/repos/github/celestiaorg/smt/badge.svg?branch=master)](https://coveralls.io/github/celestiaorg/smt?branch=master)
+[![codecov](https://codecov.io/gh/celestiaorg/smt/branch/master/graph/badge.svg?token=U3GGEDSA94)](https://codecov.io/gh/celestiaorg/smt)
 [![GoDoc](https://godoc.org/github.com/celestiaorg/smt?status.svg)](https://godoc.org/github.com/celestiaorg/smt)
 
 ## Example