Fix tutorial typo (#625)

* Fix tutorial typo

* Remove reference to `ref_backend` in docs

Author: penelopeysm

DifferentiationInterfaceTest/docs/src/tutorial.md

@@ -9,7 +9,7 @@ import ForwardDiff, Zygote
 
 ## Introduction
 
-The AD backends we want to compare are [ForwardDiff.jl](https://github.com/JuliaDiff/ForwardDiff.jl) and [Enzyme.jl](https://github.com/EnzymeAD/Enzyme.jl).
+The AD backends we want to compare are [ForwardDiff.jl](https://github.com/JuliaDiff/ForwardDiff.jl) and [Zygote.jl](https://github.com/FluxML/Zygote.jl).
 
 ```@example tuto
 backends = [AutoForwardDiff(), AutoZygote()]
@@ -60,8 +60,6 @@ test_differentiation(
 )
 ```
 
-If you are too lazy to manually specify the reference, you can also provide an AD backend as the `ref_backend` keyword argument, which will serve as the ground truth for comparison.
-
 ## Benchmarking
 
 Once you are confident that your backends give the correct answers, you probably want to compare their performance.