Fix issue when selecting single challenge does nothing.

It was not added to challenge set :(

src/main/java/world/bentobox/challenges/panel/util/SelectChallengeGUI.java

@@ -161,6 +161,7 @@ public class SelectChallengeGUI
 				}
 				else
 				{
+					this.selectedChallenges.add(challenge);
 					this.consumer.accept(true, this.selectedChallenges);
 				}