Change test-bitcoincore-rpc to mockcore in README.md (#3842)

README.md

@@ -202,7 +202,7 @@ We also try to follow a TDD (Test-Driven-Development) approach, which means we
 use tests as a way to get visibility into the code. Tests have to run fast for that
 reason so that the feedback loop between making a change, running the test and
 seeing the result is small. To facilitate that we created a mocked Bitcoin Core
-instance in [test-bitcoincore-rpc](./test-bitcoincore-rpc).
+instance in [mockcore](./crates/mockcore)
 
 Syncing
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