dev-cmd/contributions: appropriately pluralize contribution statement

[dtrodrigues]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Pluralize the contribution statement appropriately

$ brew contributions --user dtrodrigues --from 2023-03-15T01:00:00Z --to 2023-03-15T02:00:00Z --repositories=core
dtrodrigues contributed 1 times between 2023-03-15T01:00:00Z and 2023-03-15T02:00:00Z.

After:

$ brew contributions --user dtrodrigues --from 2023-03-15T01:00:00Z --to 2023-03-15T02:00:00Z --repositories=core
dtrodrigues contributed 1 time between 2023-03-15T01:00:00Z and 2023-03-15T02:00:00Z.

[BrewTestBot]

Review period will end on 2023-03-21 at 00:00:00 UTC.

[issyl0]

Thanks!

[MikeMcQuaid]

Thanks again @dtrodrigues!