Make description for "service" parameter less ambiguous.

[lmller]

The description for the "service" param states:

"The name of the ECS service to deploy to. The action will only register the task definition if no service is given."

But in this case "only" is not meant as a condition, but as a limitation... I stumbled upon this ambiguity in the documentation and it cost me an hour of my life, so I thought I might as well change it :-)

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