Fix linear operator docs

Fixes #2877 (comment)

Author: ChrisRackauckas

docs/src/semilinear/Linear.md

@@ -100,14 +100,14 @@ you will need to install some of the other libraries listed in the navigation ba
 
 ```julia
 using OrdinaryDiffEqLinear, SciMLOperators
-function update_func(A, u, p, t)
+function update_func!(A, u, p, t)
     A[1, 1] = 0
     A[2, 1] = sin(u[1])
     A[1, 2] = -1
     A[2, 2] = 0
 end
 A0 = ones(2, 2)
-A = MatrixOperator(A0, update_func = update_func)
+A = MatrixOperator(A0, update_func! = update_func!)
 u0 = ones(2)
 tspan = (0.0, 30.0)
 prob = ODEProblem(A, u0, tspan)