Merge pull request bitcoinbook#691 from vincentdnl/patch-1

Update ch04.asciidoc

ch04.asciidoc

@@ -23,7 +23,7 @@ Since the invention of public key cryptography, several suitable mathematical fu
 
 In bitcoin, we use public key cryptography to create a key pair that controls access to bitcoin. The key pair consists of a private key and--derived from it--a unique public key. The public key is used to receive funds, and the private key is used to sign transactions to spend the funds.
 
-There is a mathematical relationship between the public and the private key that allows the private key to be used to generate signatures on messages. This signature can be validated against the public key without revealing the private key.
+There is a mathematical relationship between the public and the private key that allows the private key to be used to generate signatures on messages. These signatures can be validated against the public key without revealing the private key.
 
 When spending bitcoin, the current bitcoin owner presents her public key and a signature (different each time, but created from the same private key) in a transaction to spend those bitcoin. Through the presentation of the public key and signature, everyone in the bitcoin network can verify and accept the transaction as valid, confirming that the person transferring the bitcoin owned them at the time of the transfer.