Fix build of InputOutput documentation

Author: contradict

docs/src/basics/InputOutput.md

@@ -37,6 +37,7 @@ This function takes a vector of variables that are to be considered inputs, i.e.
 The following example implements a simple first-order system with an input `u` and state `x`. The function `f` is generated using `generate_control_function`, and the function `f` is then tested with random input and state values.
 
 ```@example inputoutput
+using ModelingToolkit
 import ModelingToolkit: t_nounits as t, D_nounits as D
 @variables x(t)=0 u(t)=0 y(t)
 @parameters k = 1