[docs] Fix typo (#3157)

Author: iamdevlinph

Readme.md

@@ -38,7 +38,7 @@ Unless instructed otherwise a disconnected client will try to reconnect forever,
 
 #### Disconnection detection
 
-An heartbeat mechanism is implemented at the Engine.IO level, allowing both the server and the client to know when the other one is not responding anymore.
+A heartbeat mechanism is implemented at the Engine.IO level, allowing both the server and the client to know when the other one is not responding anymore.
 
 That functionality is achieved with timers set on both the server and the client, with timeout values (the `pingInterval` and `pingTimeout` parameters) shared during the connection handshake. Those timers require any subsequent client calls to be directed to the same server, hence the `sticky-session` requirement when using multiples nodes.