update-report: use appropriate pronoun in update message

[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?
• Have you successfully run brew man locally and committed any changes?

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Before:

You have 1 outdated formula installed.
You can update them with brew upgrade.

After

You have 1 outdated formula installed.
You can update it with brew upgrade.

[BrewTestBot]

Review period will end on 2021-02-22 at 16:37:12 UTC.

[MikeMcQuaid]

Makes sense, thanks @dtrodrigues!

[BrewTestBot]

Review period ended.