[Documentation] Update k8s walkthrough to be more clear

[BenTsai85]

When I was following https://github.com/aws/aws-app-mesh-examples/tree/master/walkthroughs/howto-k8s-http-headers, some steps were also required to set it up but not documented:

Before running ./deploy.sh:

• Install docker
• Make sure docker is running by sudo systemctl start docker
• Run the following lines to add permission:

sudo groupadd docker
sudo usermod -aG docker ${USER}

• Completely log out and log back in

After all these steps, I can successfully run ./deploy.sh.