Add extra namespace in Basic Usage example

[nilsdeppe]

Thanks for the great library!

Add the arch namespace needed to get access to avx::batch.

It looks like the README uses batch<double, 4>. If that's preferred I can update the PR to do that instead.

[JohanMabille]

Thanks for this fix. Both usages are possible (either with the size or with through the arch namespace). A huge refactoring is on-going so that users can specify explicitly the architectur instead of the size.