Allow modelingtoolkit v9 in doc builds

Author: ChrisRackauckas

docs/Project.toml

@@ -60,7 +60,7 @@ FiniteDiff = "2"
 ForwardDiff = "0.10, 1"
 IncompleteLU = "0.2"
 JLD2 = "0.4, 0.5.1"
-ModelingToolkit = "10"
+ModelingToolkit = "9, 10"
 NonlinearSolve = "3.15, 4"
 ODEProblemLibrary = "0.1"
 Optimization = "3, 4"