Update CONTRIBUTING.md (#638)
Update CONTRIBUTING.md (#638)

Fixed label for good first issue to actual used value.
alekseysidorov/exonum,exonum/exonum,exonum/exonum,alekseysidorov/exonum,alekseysidorov/exonum,exonum/exonum,exonum/exonum,alekseysidorov/exonum