rename Polkadot to Substrate in the license header via following four commands (#614)

git grep -l 'This file is part of Polkadot' | xargs sed -i 's/This file is part of Polkadot/This file is part of Substrate/g'
    git grep -l 'Polkadot is free software' | xargs sed -i 's/Polkadot is free software/Substrate is free software/g'
    git grep -l 'Polkadot is distributed in the hope' | xargs sed -i 's/Polkadot is distributed in the hope/Substrate is distributed in the hope/g'
    git grep -l 'along with Polkadot' | xargs sed -i 's/along with Polkadot/along with Substrate/g'