fix typeo

Author: baumgold

src/arraytypes/struct.jl

@@ -78,7 +78,7 @@ struct ToStruct{T,i,A} <: AbstractVector{T}
 end
 
 function ToStruct(x::A, j::Integer, hasmissing::Bool=false) where {A}
-    AT = fieldtype(eltype(A), k)
+    AT = fieldtype(eltype(A), j)
     T = hasmissing ? Union{Missing,AT} : AT
     ToStruct{T,j,A}(x)
 end