doc: Change #bitcoin-dev IRC channel to #bitcoin-core-dev

Split off Bitcoin Core project development discussion from the general
Bitcoin development channel.

README.md

@@ -36,7 +36,7 @@ The developer [mailing list](https://lists.linuxfoundation.org/mailman/listinfo/
 should be used to discuss complicated or controversial changes before working
 on a patch set.
 
-Developer IRC can be found on Freenode at #bitcoin-dev.
+Developer IRC can be found on Freenode at #bitcoin-core-dev.
 
 Testing
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