Banana Labs

Fix broken link in reentrancy section from chapter 09

1

In the further reading on [reentrancy](https://github.com/ethereumbook/ethereumbook/blob/develop/09smart-contracts-security.asciidoc#reentrancy) attack the link [Ethereum Smart Contract Best Practices](http://bit.ly/2ERDMxV) is dead [404]. Please Remove or update it.