doc: update labels in CONTRIBUTING.md
doc: update labels in CONTRIBUTING.md

peercoin/peercoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,DigitalPandacoin/pandacoin,peercoin/peercoin,peercoin/peercoin