Fix exception text for bad duration value

It said it expected a string or number of $units,
but really if it's a number it always has to be
milliseconds.

config/src/main/java/com/typesafe/config/impl/SimpleConfig.java

@@ -408,7 +408,7 @@ final class SimpleConfig implements Config, MergeableValue, Serializable {
                 l.add(n);
             } else {
                 throw new ConfigException.WrongType(v.origin(), path,
-                        "duration string or number of " + unit.name(),
+                        "duration string or number of milliseconds",
                         v.valueType().name());
             }
         }