fix:Lesson_12 bug in stack.java

Author: txtran224

lesson_12/structs_java/structs_app/src/main/java/com/codedifferently/lesson12/Stack.java

@@ -27,6 +27,6 @@ public int peek() {
   }
 
   public boolean isEmpty() {
-    return top == null
+    return top ==null;
   }
 }