Update Main.java

class BookTopology is a thread as it is implementing Runnable.  It has been defined and overridden inside the BookTopology class but the call to this thread should have been made from Main class by calling a start() method.  even though the thread was instantiated in the Main class but instead of start(); it used run();  method. By using run(); it does not start another thread but starts the execution from the same single main method's thread. By using start(); we could have truly achieved multithreading as it starts another thread.