fix minor typo (JuliaGaussianProcesses#313)

Author: st--

docs/src/index.md

@@ -7,6 +7,6 @@ The main goals of this package are:
 - **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.
 
-This package builds on 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 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).
 
 See the [User guide](@ref) for a brief introduction.