Update index.md (#435)

* Update index.md

Fixed a typo.

* Apply suggestions from code review

Co-authored-by: st-- <st--@users.noreply.github.com>

Author: pitmonticone

docs/src/index.md

@@ -5,8 +5,8 @@ It aims at providing a flexible framework for creating kernels and manipulating
 The main goals of this package are:
 - **Flexibility**: operations between kernels should be fluid and easy without breaking.
 - **Plug-and-play**: including the kernels before/after other steps should be straightforward.
-- **Automatic Differentation** compatibility: all kernel functions which _ought_ to be differentiable using AD packages like [ForwardDiff.jl](https://github.com/JuliaDiff/ForwardDiff.jl) or [Zygote.jl](https://github.com/FluxML/Zygote.jl) _should_ be.
+- **Automatic Differentiation compatibility**: all kernel functions which _ought_ to be differentiable using AD packages like [ForwardDiff.jl](https://github.com/JuliaDiff/ForwardDiff.jl) or [Zygote.jl](https://github.com/FluxML/Zygote.jl) _should_ be.
 
-This package builds on top of lots of excellent existing work in packages such as [MLKernels.jl](https://github.com/trthatcher/MLKernels.jl), [Stheno.jl](https://github.com/willtebbutt/Stheno.jl), [GaussianProcesses.jl](https://github.com/STOR-i/GaussianProcesses.jl), and [AugmentedGaussianProcesses.jl](https://github.com/theogf/AugmentedGaussianProcesses.jl).
+This package replaces the now-defunct [MLKernels.jl](https://github.com/trthatcher/MLKernels.jl). It incorporates lots of excellent existing work from packages such as [GaussianProcesses.jl](https://github.com/STOR-i/GaussianProcesses.jl), and is used in downstream packages such as [AbstractGPs.jl](https://github.com/JuliaGaussianProcesses/AbstractGPs.jl), [ApproximateGPs.jl](https://github.com/JuliaGaussianProcesses/ApproximateGPs.jl), [Stheno.jl](https://github.com/willtebbutt/Stheno.jl), and [AugmentedGaussianProcesses.jl](https://github.com/theogf/AugmentedGaussianProcesses.jl).
 
 See the [User guide](@ref) for a brief introduction.