Fix Eland Docker image name

[pquentin]

When we instructed users to build the image, the tag was elastic/eland, but now we should refer to the docker.elastic.co image tag instead.

Thanks to @itay-ct for the report.

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @pquentin? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[davidkyle]

LGTM

[lcawl]

LGTM, thanks!

Please note that if you want this automatically backported across the doc versions, you'll need to add backport-8.15, backport-8.14, backport-8.13, backport-8.12 labels, for example

[pquentin]

I don't have the necessary permissions unfortunately. Backporting all the way back to 8.9 would be ideal.