doc: Update bucketExists in API.md (#821)
doc: Update bucketExists in API.md (#821)

Since `bucketExists` was [promisified](https://github.com/minio/minio-js/blob/518e5a8815dd579f3fcd2d2a4f4cb2599ad3d580/src/main/minio.js#L2115), it should be reflected in the API document.
minio/minio-js