Update ec2.md with note on failing to install drivers

[raybellwaves]

Just came across this error while connecting to an EC2 instance. The fix was suggested at https://forums.developer.nvidia.com/t/nvidia-gpu-optimized-ami-is-missing-drivers-and-cant-run-the-pytorch-ngc-dockerfile/276492/2 and made a note here in case others come across this.

[jacobtomlinson]

Thanks @raybellwaves