Update gpu_tutorial.md (#544)
Update gpu_tutorial.md (#544)

It's not obvious that the y-axis is log-scale and I believe it's easy to miss...
I edited the text so it would be easier to understand
fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3,fastai/course-v3