Update splits.md `tfds` Splits guide
Update splits.md `tfds` Splits guide

Minor improvements: define two params as per `tfds.core.NamedSplit` API docs for better UX (namely, `k` for even subsplits and `weighted` for proportional subsplits), change "TFDS" to "TensorFlow Datasets" for consistency, split one complex sentence into two simpler ones, change "Ds" to `ds` in a comment
tensorflow/datasets,tensorflow/datasets,tensorflow/datasets,tensorflow/datasets,tensorflow/datasets