fixup! refactor: update implementation of discrete save interface

Author: AayushSabharwal

test/mtkparameters.jl

@@ -299,7 +299,7 @@ with_updated_parameter_timeseries_values(
     sys, ps, 1 => ModelingToolkit.NestedGetIndex(([5.0, 10.0],)))
 @test ps.discrete[1][1] == [5.0, 10.0]
 with_updated_parameter_timeseries_values(
-    ps, 1 => ModelingToolkit.NestedGetIndex(([3.0, 30.0],)),
+    sys, ps, 1 => ModelingToolkit.NestedGetIndex(([3.0, 30.0],)),
     2 => ModelingToolkit.NestedGetIndex(([4.0, 40.0],)))
 @test ps.discrete[1][1] == [3.0, 30.0]
 @test ps.discrete[2][1] == [4.0, 40.0]