Update build-linux.md
Update build-linux.md

Fixed the PPA instructions. The BitcoinClassic PPA has the BDB 4.8 necessary to build, and this way people don't add the Bitcoin Core PPA which could lead to them somehow installing Bitcoin Core by accident.
zander/bitcoinclassic,zander/bitcoinclassic,zander/bitcoinclassic,adpg211/bitcoin-master,deadalnix/bitcoin,deadalnix/bitcoin,deadalnix/bitcoin,bitcoinclassic/bitcoinclassic,adpg211/bitcoin-master,adpg211/bitcoin-master,adpg211/bitcoin-master,deadalnix/bitcoin,bitcoinclassic/bitcoinclassic,zander/bitcoinclassic,bitcoinclassic/bitcoinclassic,zander/bitcoinclassic,deadalnix/bitcoin,deadalnix/bitcoin,bitcoinclassic/bitcoinclassic,adpg211/bitcoin-master,adpg211/bitcoin-master,bitcoinclassic/bitcoinclassic,bitcoinclassic/bitcoinclassic,zander/bitcoinclassic