Fix EnsembleSummary plot recipe idxs kwarg

`EnsembleSummary`'s plot recipe refered to the variable indices as `trajectories`, likely typo from the `EnsembleSimulation` recipe above where that nomenclature made sense. 

Fixed it to use `idxs`, allowing users to select which variables to plot the summaries of, in line with the documentation.

src/ensemble/ensemble_solutions.jl

@@ -158,7 +158,7 @@ end
 end
 
 @recipe function f(sim::EnsembleSummary;
-        trajectories = sim.u.u[1] isa AbstractArray ? eachindex(sim.u.u[1]) :
+        idxs = sim.u.u[1] isa AbstractArray ? eachindex(sim.u.u[1]) :
                        1,
         error_style = :ribbon, ci_type = :quantile)
     if ci_type == :SEM
@@ -192,7 +192,7 @@ end
     else
         error("ci_type choice not valid. Must be `:SEM` or `:quantile`")
     end
-    for i in trajectories
+    for i in idxs
         @series begin
             legend --> false
             linewidth --> 3