Fix typo in common_first_steps.jl

Author: vavrines

docs/common_first_steps.jl

@@ -18,7 +18,7 @@ function first_steps(name, solver)
 
     function lorenz!(du, u, p, t)
         du[1] = 10.0 * (u[2] - u[1])
-        u[2] = u[1] * (28.0 - u[3]) - u[2]
+        du[2] = u[1] * (28.0 - u[3]) - u[2]
         du[3] = u[1] * u[2] - (8 / 3) * u[3]
     end
     u0 = [1.0; 0.0; 0.0]