Update README.md

Just a typo

README.md

@@ -4,7 +4,7 @@ PyBitmessage
 Bitmessage is a P2P communications protocol used to send encrypted messages to
 another person or to many subscribers. It is decentralized and trustless,
 meaning that you need-not inherently trust any entities like root certificate
-authorities. It uses strong authentication which means that the sender of a
+authorities. It uses strong authentication, which means that the sender of a
 message cannot be spoofed, and it aims to hide "non-content" data, like the
 sender and receiver of messages, from passive eavesdroppers like those running
 warrantless wiretapping programs.