Fix outdated HasSizeAtLeast method name in Guard docs

[Benziza]

The Collections table still lists HasSizeAtLeast, but that one was renamed to HasSizeGreaterThanOrEqualTo a while back, so copying the row straight out of the table doesn't compile:

error CS0117: 'Guard' does not contain a definition for 'HasSizeAtLeast'

Left the description alone since it already said the right thing, just swapped "at least" for the wording used by the row below it.

[learn-build-service-prod]

Learn Build status updates of commit a49f6c2:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/diagnostics/Guard.md	✅Succeeded	View

For more details, please refer to the build report.

[learn-build-service-prod]

PoliCheck Scan Report

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