replace 'a' with 'an'

[A or An](https://www.a-or-an.com/a_an/axe)

factory-method/src/test/java/com/iluwatar/factory/method/FactoryMethodTest.java

@@ -53,7 +53,7 @@ public class FactoryMethodTest {
   }
 
   /**
-   * Testing {@link OrcBlacksmith} to produce a AXE asserting that the Weapon is an instance
+   * Testing {@link OrcBlacksmith} to produce an AXE asserting that the Weapon is an instance
    *  of {@link OrcWeapon}.
    */
   @Test