Fix return type of makeSingleInstance

docs/api/app.md

@@ -667,6 +667,8 @@ app.setJumpList([
   * `argv` String[] - An array of the second instance's command line arguments
   * `workingDirectory` String - The second instance's working directory
 
+Returns `Boolean`.
+
 This method makes your application a Single Instance Application - instead of
 allowing multiple instances of your app to run, this will ensure that only a
 single instance of your app is running, and other instances signal this