docs: 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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[amazreech]

Hi @lmller,

Thank you for your Patience. We appreciate your contribution to the repository and will be working to review the changes in the Pull Request.

In the mean time please ensure below steps are taken care of in your PR:

1. Verify if PR follows semantic pull request conventions.
   Example: change PR title to follow conventions.

2. Please be sure to rebase and resolve merge any conflicts on the PR, if necessary.