MainApp.java: Fix typo Address Book -> AddressBook

When launching AB3 from terminal using java -jar, the start
of logging says 'Starting AddressBook' while the end says
'Stopping Address Book'.

Let's fix this inconsistency by changing it to 'Stopping 
AddressBook' in MainApp.java.

src/main/java/seedu/address/MainApp.java

@@ -176,7 +176,7 @@ public void start(Stage primaryStage) {
 
  @Override
  public void stop() {
- logger.info("============================ [ Stopping Address Book ] =============================");
+ logger.info("============================ [ Stopping AddressBook ] =============================");
  try {
  storage.saveUserPrefs(model.getUserPrefs());
  } catch (IOException e) {