Update ECS Fargate Integration documentation

[vhiairrassary]

What does this PR do?

Fix the documentation at https://docs.datadoghq.com/integrations/ecs_fargate/ for accounts using datadog.eu.

Motivation

I was not able to use ECS Fargate integration without this.

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[ChristineTChen]

Hi @vhiairrassary! Thanks for making the PR and bringing this discrepancy to our attention. I've added some labels and also requested our docs team to take a look.

[apigirl]

thanks so much for adding this @vhiairrassary!