Updated the boot disk size based on GCP advice
Updated the boot disk size based on GCP advice

It is noted in the GCP [Storage Options](http://google.com) docs that "Performance on persistent disks is predictable, scales linearly, and is directly related to boot disk size. A boot disk smaller than 200GB will result in a poor experience."
fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3