Fix ECS_RESERVED_MEMORY unit in README
Fix ECS_RESERVED_MEMORY unit in README

Use proper unit (`MiB`) for `ECS_RESERVED_MEMORY` in the README.

swipely/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,aws/amazon-ecs-agent,swipely/amazon-ecs-agent,swipely/amazon-ecs-agent,aws/amazon-ecs-agent,aws/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,kiranmeduri/amazon-ecs-agent,swipely/amazon-ecs-agent,aws/amazon-ecs-agent