NES-24660 - Update wording on Open Rate

[tschant]

What Changed

• updated wording on Open Rate to call out non-prefetch opens only

How To Test or Verify

• Verify wording on /support-docs/content/docs/reporting/enterprise-metrics-definitions.md.

PR Checklist

All PRs Checklist

• Give your pull request a meaningful name.
• Use lowercase filenames.
• Apply at least one team label according to which team is the content expert (ie. team-FE or team-SAZ)
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Content Changes Checklist

• Check that your article looks correct in the preview here or in a Netlify deploy preview.
• Check the links in your article.
• Check the images in your article (if there are any)
• Check to make sure you are using markdown appropriately as outlined in examples/article.md in the root of the project directory and on the momentum doc's preface article
• Check to make sure the Copy and Tone Guidelines are followed.

[netlify]

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