Rename mystery field in JoinGame

proxy/src/main/java/com/velocitypowered/proxy/protocol/packet/JoinGame.java

@@ -18,7 +18,7 @@ public class JoinGame implements MinecraftPacket {
   private @Nullable String levelType;
   private int viewDistance; //1.14+
   private boolean reducedDebugInfo;
-  private boolean mystery;
+  private boolean showRespawnScreen;
 
   public int getEntityId() {
     return entityId;
@@ -130,7 +130,7 @@ public class JoinGame implements MinecraftPacket {
       this.reducedDebugInfo = buf.readBoolean();
     }
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
-      this.mystery = buf.readBoolean();
+      this.showRespawnScreen = buf.readBoolean();
     }
   }
 
@@ -161,7 +161,7 @@ public class JoinGame implements MinecraftPacket {
       buf.writeBoolean(reducedDebugInfo);
     }
     if (version.compareTo(ProtocolVersion.MINECRAFT_1_15) >= 0) {
-      buf.writeBoolean(mystery);
+      buf.writeBoolean(showRespawnScreen);
     }
   }