fix typo

Author: vyudu

lib/ImplicitDiscreteSolve/src/solve.jl

@@ -34,7 +34,7 @@ end
 function _initialize_dae!(integrator, prob::ImplicitDiscreteProblem,
         alg::DefaultInit, x::Union{Val{true}, Val{false}})
     isnothing(prob.u0) && return
-    atol = one(eltype(u0)) * 1e-12
+    atol = one(eltype(prob.u0)) * 1e-12
     if SciMLBase.has_initializeprob(prob.f)
         _initialize_dae!(integrator, prob,
                          OverrideInit(atol), x)