update arXiv logo
arXiv has a new logo: https://arxiv.org/about/brand_use

I expect that the [small logomark](https://cornell.box.com/s/cleq2ex1wra4sfsj94tlqrw9z5oecg6b) is the most appropriate, but it may be best to ask them:
> Have questions? Need a different color? Please reach out to nextgen@arxiv.org with your request and include a link to your application.

In particular, they may have a monochrome version already available.