change error message in ObjectUtils#nonNull

Author: GliczDev

slime-loader/src/main/java/net/roxymc/slime/util/ObjectUtils.java

@@ -7,6 +7,6 @@ private ObjectUtils() {
     }
 
     public static <T> T nonNull(T obj, String name) {
-        return Objects.requireNonNull(obj, name + " deserializer == null");
+        return Objects.requireNonNull(obj, name + " == null");
     }
 }