digitalocean: Fix instance size name
digitalocean: Fix instance size name

Before, the size descriptions listed the smallest instance size as `.5mb`
instead of `512mb`. Since DigitalOcean's API doesn't recognize this size, VMs
would fail to boot if their size was determined to be .5mb based on resource
requirements in the Quilt spec.

LuiseV/quilt,kayousterhout/quilt,LuiseV/quilt,ejj/quilt-dev,kklin/quilt,kklin/kelda,LuiseV/quilt,kklin/quilt,quilt/quilt,kklin/quilt,NetSys/quilt,kklin/kelda,LuiseV/quilt,kayousterhout/quilt,NetSys/quilt,kelda/kelda,ejj/quilt-dev,NetSys/quilt,kayousterhout/quilt,NetSys/quilt,quilt/quilt,kklin/kelda,kelda/kelda,quilt/quilt,kklin/kelda,LuiseV/quilt,kelda/kelda,ejj/quilt-dev,ejj/quilt-dev,quilt/quilt,kklin/quilt,kelda/kelda,NetSys/quilt,kayousterhout/quilt