fix typings for NConsumer

[MrSchneepflug]

This PR will add missing type string.

See lib/librdkafka/NConsumer.js:

/**
 * creates a new consumer instance
 * @param {string|Array} topics - topic or topics to subscribe to
 * @param {object} config - configuration object
 */
constructor(topics, config = { options: {}, health: {} }) {
  [...]
  this.topics = Array.isArray(topics) ? topics : [topics];
  [...]
}