dry run output: Println -> Printf as intended

[gtklocker]

Output before:

~/go/bin(master*) » sudo ./embiggen-disk -dry-run /
[dry-run] would've run sfdisk -f to set new partition table
[dry-run] would've run %v %v /usr/bin/btrfs [btrfs filesystem resize max /]
No changes made.

Output after:

~/go/bin(master*) » sudo ./embiggen-disk -dry-run /
[dry-run] would've run sfdisk -f to set new partition table
[dry-run] would've run /usr/bin/btrfs [btrfs filesystem resize max /]
No changes made.

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[gtklocker]

I signed it!

[googlebot]

CLAs look good, thanks!

[bradfitz]

Thanks!