modernize definition of t

Author: jaakkor2

README.md

@@ -48,11 +48,11 @@ The following is the [RC Circuit Demonstration](https://docs.sciml.ai/ModelingTo
 using ModelingToolkit, OrdinaryDiffEq, Plots
 using ModelingToolkitStandardLibrary.Electrical
 using ModelingToolkitStandardLibrary.Blocks: Constant
+using ModelingToolkit: t_nounits as t
 
 R = 1.0
 C = 1.0
 V = 1.0
-@variables t
 systems = @named begin
     resistor = Resistor(R = R)
     capacitor = Capacitor(C = C, v = 0.0)