Update README.md (#140)

Corrected of spelling of the.

Author: digidol

samples/RealtimeSignIn/README.md

@@ -130,7 +130,7 @@ The body of the request is a JSON object with two properties:
 1. `target`: The target method you want to call in clients.
 2. `arguments`: an array of arguments you want to send to clients.
 
-The service authenticates REST call using JWT token, when you're generating the JWT token, make sure the audience is same as the REST API url and use the access key in SignalR service connection string as th secret key. Then put it in authentication header:
+The service authenticates REST call using JWT token, when you're generating the JWT token, make sure the audience is same as the REST API url and use the access key in SignalR service connection string as the secret key. Then put it in authentication header:
 
 ```
 Authorization: Bearer <jwt_token>