Fix typo (#37)

ch01.asciidoc

@@ -23,7 +23,7 @@ The first response to Satoshi Nakamotos initial publication of the Bitcoin white
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 We very, very much need such a system, but the way I understand your proposal, it does not seem to scale to the required size.
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-While it seemed as if James A. Donald just refered to keeping the set of unspend transaction outputs (UTXOs) it quickly became clear that also verifying and storing that many transactions would become infeasible for any blockchain.
+While it seemed as if James A. Donald just refered to keeping the set of unspent transaction outputs (UTXOs) it quickly became clear that also verifying and storing that many transactions would become infeasible for any blockchain.
 
 A key requirement for a second layer protocol such as lightning (and as will be decribed in greater depth later in this book) is the ability to sequence transactions external to the blockchain. In the first verisons of Bitcoin, Satoshi Nakamoto recognised this and introduced a data field called `nSequence` into the input transaction data.
 The `nSequence`  was intended to allow users to transmit updated versions of a transaction to the network, changing the outputs of a transaction, effectively creating a payment channel.