Correct the Docker buildx command in SDK 1.25.0 upgrade doc

[semihbkgr]

Description of the change:

The arg for the docker buildx build command was missing in the documentation, resulting in the following error:

ERROR: "docker buildx build" requires exactly 1 argument.

I have added the arg as the current directory:
docker buildx build --push --platform=$(PLATFORMS) --tag ${IMG} -f Dockerfile.cross .

Motivation for the change:

I identified this subtle issue while upgrading an operator to version 1.25.0.

Checklist

If the pull request includes user-facing changes, extra documentation is required:

• Add a new changelog fragment in changelog/fragments (see changelog/fragments/00-template.yaml)
• Add or update relevant sections of the docs website in website/content/en/docs

[openshift-bot]

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