Fix typo for alpha*, explain partition_id sorting

datasets/flwr_datasets/partitioner/distribution_partitioner.py

@@ -41,11 +41,13 @@ class DistributionPartitioner(Partitioner):  # pylint: disable=R0902
     ( `num_unique_labels`, ---------------------------------------------------- ),
                                           `num_unique_labels`
     the label_id at the i'th row is assigned to the partition_id based on the formula:
-        partition_id = alpha + beta
+        partition_id = <alpha + beta>
     where,
+        <.> denotes the reindexed sequence of partition_ids in monotone increasing
+            order for all j's
         alpha* = (i - num_unique_labels_per_partition + 1) \
-                 + (j % num_unique_labels_per_partition)
-        alpha = alpha* + (alpha* > 0 ? 0 : num_unique_labels)
+                 + (j % num_unique_labels_per_partition),
+        alpha = alpha* + (alpha* >= 0 ? 0 : num_unique_labels),
         beta = num_unique_labels * (j // num_unique_labels_per_partition)
     and j in {0, 1, 2, ..., `num_columns`}. Each list representing the partition_ids for
     the i'th row is sorted in ascending order. So, for a dataset with 10 unique labels