Update FighterTest.java

fixed  compilation error

Author: NotsOverflow

exercises/concept/wizards-and-warriors/src/test/java/FighterTest.java

@@ -213,14 +213,14 @@ void testWizardsDamagePointsAfterPreparingSpell() {
 
     private static class VulnerableFighter extends Fighter {
         @Override
-        boolean isVulnerable() {
+        public boolean isVulnerable() {
             return true;
         }
     }
 
     private static class InvulnerableFighter extends Fighter {
         @Override
-        boolean isVulnerable() {
+        public boolean isVulnerable() {
             return false;
         }
     }