docs(shard): add .cache to GuildManager in fetchClientValue exa… (#3857)

src/sharding/Shard.js

@@ -220,7 +220,7 @@ class Shard extends EventEmitter {
    * @param {string} prop Name of the client property to get, using periods for nesting
    * @returns {Promise<*>}
    * @example
-   * shard.fetchClientValue('guilds.size')
+   * shard.fetchClientValue('guilds.cache.size')
    *   .then(count => console.log(`${count} guilds in shard ${shard.id}`))
    *   .catch(console.error);
    */