Update description of ascii format
Makes the changes that were proposed in https://git.glasklar.is/sigsum/project/documentation/-/blob/main/proposals/2022-10-restructure-ascii-format.md
Edited by Niels Möller
Makes the changes that were proposed in https://git.glasklar.is/sigsum/project/documentation/-/blob/main/proposals/2022-10-restructure-ascii-format.md