cmd/part: correct alignment of partition list

When running 'part list' for an ISO partition the numbers are not under the
labels.

Correct the alignment of the ISO partition list. With the patch the
output looks like:

    Part   Start     Sect x Size Type
      1     3720     5024    512 U-Boot

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>