Fix example in Arrow.Writer (#480)

Author: guilhermebodin

src/write.jl

@@ -104,7 +104,7 @@ julia> open(Arrow.Writer, tempname()) do writer
            partition1 = (col1 = [1, 2], col2 = ["A", "B"])
            Arrow.write(writer, partition1)
            partition2 = (col1 = [3, 4], col2 = ["C", "D"])
-           Arrow.write(writer, partition1)
+           Arrow.write(writer, partition2)
        end
 ```
 """