Add incrmntal to who-is-using.md

[scravy]

This adds incrmntal to the list of users of spark-on-k8s-operator. We are using the operator in production on EKS to operate spark without the needs for Hadoop and using cloud filesystems directly (using s3a output committer).

Best

[google-cla]

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[scravy]

@googlebot I signed it!