Fix format typo in Structure.dump()

[laomaiweng]

'{#:8X}'.format(val) yields a KeyError exception complaining about the #.

Alternate-form # format specifier should come after the colon, so this PR fixes this which seems to be a mere typo.

[malwarefrank]

I suggest changing "8" to "10" in that one line, since the prefix (0X) will use two characters. That would also makes it align better in the output.