Apply suggestion from @petrelharp

Co-authored-by: Peter Ralph <petrel.harp@gmail.com>

Author: apragsdale

python/tskit/trees.py

@@ -10995,7 +10995,7 @@ def ld_matrix(
         :math:`\widehat{\pi_2}`  n          n               "pi2_unbiased"
         ======================= ========== ================ ==============
 
-        :param list sample_sets: A list of lists of sample Node IDs, specifying
+        :param list sample_sets: A list, or a list of lists of sample node IDs, specifying
             the groups of nodes to compute the statistic with. Defaults to all
             samples grouped by population.
         :param str mode: A string giving the "type" of the statistic to be