Add line explaining the "%sort" test.

(grafted from 1ea833ecaf5a9d43a886e9e73b4e2551d0d5b548)

Objects/listsort.txt

@@ -60,6 +60,7 @@ Comparison with Python's Samplesort Hybrid
       /sort: ascending data
       3sort: ascending, then 3 random exchanges
       +sort: ascending, then 10 random at the end
+      %sort: ascending, then randomly replace 1% of elements w/ random values
       ~sort: many duplicates
       =sort: all equal
       !sort: worst case scenario