fixed another typo in the Bitcoin paper

Hi, I've stumbled upon another typo in the transcribed Bitcoin paper while proofreading the text for our ePub & MOBI version of the paper from Liberty's Legacy Edition: "Bitcoin: A Peer-to-Peer Electronic Cash System". No rights reserved. Happy reading, sharing, editing and selling!

[Bitcoin: A Peer-to-Peer Electronic Cash System](http://www.readliberty.org/books/bitcoin-a-peer-to-peer-electronic-cash-system)