add quotes surrounding $(brew --cache) in help for cleanup command

[yurikoles]

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[reitermarkus]

You'll need to run brew man to update the manpage.

[yurikoles]

@reitermarkus please help me with that.

[scpeters]

Can you run the command brew man and commit the resulting changed files?

[yurikoles]

done

[MikeMcQuaid]

Thanks @yurikoles!