Fix typo in MetricRegistriesImpl#create warning message

  @Override
  public RatisMetricRegistry create(MetricRegistryInfo info) {
    return registries.put(info, () -> {
      if (reporterRegistrations.isEmpty()) {
        LOG.warn(
            "First MetricRegistry has been created without registering reporters. You may need to call" +
                " MetricRegistries.global().addReporterRegistration(...) before.");
      }
      RatisMetricRegistry registry = factory.create(info);
      reporterRegistrations.forEach(reg -> reg.accept(registry));
      return registry;
    });
  }