fix: convert builder factory method to static

Author: hbruch

src/main/java/org/opentripplanner/routing/api/request/preference/RoutingPreferences.java

@@ -54,7 +54,7 @@ private RoutingPreferences(Builder builder) {
     this.itineraryFilter = requireNonNull(builder.itineraryFilter());
   }
 
-  public Builder of() {
+  public static Builder of() {
     return DEFAULT.copyOf();
   }