formula_cellar_checks: fix alignment when listing files

[carlocab]

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The extra spacing came from before we used squiggly heredocs. Now that
we do, they're no longer necessary, and produce misaligned output.

[BrewTestBot]

Review period will end on 2021-10-12 at 00:00:00 UTC.

[BrewTestBot]

Review period skipped due to critical label.