doc: disk-utils/*: Fix some warnings from "mandoc -T lint"

doc: disk-utils/*: Fix some warnings from "mandoc -T lint"

mandoc: ./disk-utils/fsck.minix.8:123:2: WARNING: skipping paragraph macro: PP empty

mandoc: ./disk-utils/isosize.8:8:2: WARNING: skipping paragraph macro: PP after SH

mandoc: ./disk-utils/sfdisk.8:262:4: STYLE: unterminated quoted argument

mandoc: ./disk-utils/swaplabel.8:57:2: WARNING: skipping paragraph macro: PP empty

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  No change in the output from "nroff" or "groff".

Signed-off-by: Bjarni Ingi Gislason <bjarniig@rhi.hi.is>
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