Update understand-usage-details-fields.md

[charliewei0716]

Add SavingsPlan as a value that appears in the PricingModel field.

[prmerger-automator[bot]]

@charliewei0716 : Thanks for your contribution! The author(s) have been notified to review your proposed change.

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[Court72]

@bandersmsft

Can you review the proposed changes?

When the changes are ready for publication, add a #sign-off comment to signal that the PR is ready for the review team to merge.

label:"aq-pr-triaged"

@MicrosoftDocs/public-repo-pr-review-team

[bandersmsft]

@charliewei0716 I'll verify your update is good with the Savings Plan enginnering team. If good, we'll merge soon.

[bandersmsft]

sign-off