Add context for installing corrplot

[maleonard96]

With a fresh install of R corrplot will not be installed, and it is not immediately obvious from the docs why this issue is happening or how to resolve it.

[learn-build-service-prod]

Learn Build status updates of commit 5b8b203:

✅ Validation status: passed

File	Status	Preview URL	Details
powerbi-docs/visuals/service-r-visuals.md	✅Succeeded

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[Jak-MS]

@mihart
Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[learn-build-service-prod]

Learn Build status updates of commit 484b8d8:

✅ Validation status: passed

File	Status	Preview URL	Details
powerbi-docs/visuals/service-r-visuals.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[github-actions]

This pull request has been inactive for 14 days. If you're done making changes, don't forget to sign off. See the contributor guide for instructions. If you're still working and want to stop these notifications, apply the "keep-open" label to your PR. However, we don't advise long-running PRs due to the risk of merge conflicts. We'll begin auto-closing stale PRs in September 2021. Thank you!
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[prmerger-automator]

@maleonard96 : Thanks for your contribution! The author(s) have been notified to review your proposed change. @BlairJArmstrong