Update src/basekernels/matern.jl

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: st--

src/basekernels/matern.jl

@@ -43,7 +43,7 @@ _get_ν(k::MaternKernel) = only(k.ν)
 function ChainRulesCore.rrule(::typeof(_get_ν), k::T) where {T<:MaternKernel}
     function _get_ν_pullback(Δ)
         dν = ChainRulesCore.@not_implemented("Derivatives w.r.t. ν are not implemented.")
-        return Tangent{T}(ν=dν, metric=NoTangent())
+        return Tangent{T}(; ν=dν, metric=NoTangent())
     end
     return _get_ν(k), _get_ν_pullback
 end