should use "=" to assign value rather than "-"

Change-Id: I555a3df45e56e33838613eb5766a4accc6f08342
Signed-off-by: Frank Han <[email protected]>

fabric-client/lib/EventHub.js

@@ -77,7 +77,7 @@ var EventHub = class {
 		// grpc event client interface
 		this._client = null;
 		// grpc chat streaming interface
-		this.call - null;
+		this.call = null;
 		// fabric connection state of this eventhub
 		this.connected = false;
 	}