Update index.md

Just added the import of jacobian from AbstractDifferentiation.jl

Author: mattiasvillani

docs/src/index.md

@@ -14,7 +14,7 @@ Please see the AbstractDifferentiation.jl docs for detailed usage.
 ```@jldoctest
 julia> using Diffractor: DiffractorForwardBackend
 
-julia> using AbstractDifferentiation: derivative
+julia> using AbstractDifferentiation: derivative, jacobian
 
 julia> derivative(DiffractorForwardBackend(), +, 1.5, 10.0)
 (1.0, 1.0)
@@ -31,4 +31,4 @@ julia> jacobian(DiffractorForwardBackend(), identity, [1.5, 2.5, 10.0]) |> only
  1.0  0.0  0.0
  0.0  1.0  0.0
  0.0  0.0  1.0
-```
+```