Correction to DORA deployment frequency definition

What does this MR do?

In the Analytics definitions documentation, the definition for DORA Deployment Frequency was slightly incorrect. This corrects it so it better reflects the official definition.

This is a preliminary step toward correcting the DORA API deployment_frequency metric. As noted by Google (emphasis added): "It would be simple and straightforward to show daily deployment volume or to grab the average number of deployments per [time period], but the metric is deployment frequency, not volume."

Conformity

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