Fix documentation example for Arrow.Writer (#479)

Author: guilhermebodin

src/write.jl

@@ -101,7 +101,7 @@ It's also possible to automatically close the Writer using a do-block:
 
 ```julia
 julia> open(Arrow.Writer, tempname()) do writer
-           partition2 = (col1 = [1, 2], col2 = ["A", "B"])
+           partition1 = (col1 = [1, 2], col2 = ["A", "B"])
            Arrow.write(writer, partition1)
            partition2 = (col1 = [3, 4], col2 = ["C", "D"])
            Arrow.write(writer, partition1)