Improve awkward wording of Backing Up App

[carter-thaxton]

Previously, it would log:
==> Backing App 'GitX.app' up to '/opt/homebrew/Caskroom/gitx/1.4/GitX.app'

Now it logs:
==> Backing Up App 'GitX.app' to '/opt/homebrew/Caskroom/gitx/1.4/GitX.app'

The previous awkward wording has been a pet peeve of mine for several years, so I finally created a PR to improve it. :)

I considered using the lower-case "Backing up App ...", but I think the upper-case "Backing Up" looks better in the surrounding context, where many other words are already capitalized.

Existing tests are modified with this PR to match the new wording.

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[p-linnane]

Makes sense. Thanks.