Make builder value private

Author: TBlueF

src/main/java/de/bluecolored/bluemap/api/markers/MarkerSet.java

@@ -241,7 +241,7 @@ public static class Builder {
 
         private String label;
         private Boolean toggleable, defaultHidden;
-        Integer sorting;
+        private Integer sorting;
 
         /**
          * Sets the label of the {@link MarkerSet}.