Fix typo `<<` in User data example of page `Launching an Amazon ECS Container Instance`

[Ernest Chiang]

One example shows cat <<'EOF' >> /etc/ecs/ecs.config which should be cat <<'EOF' >> /etc/ecs/ecs.config. Would like to fix it to avoid reader miss configuration.

[Joel Brandenburg]

Thanks for your contribution, this has been added to the documentation.