manual/README.md: stop mentionning the DVI manual
manual/README.md: stop mentionning the DVI manual

This is a follow-up to commit 2f0d65c7ee522f77765338b5889dc4bbc0979607
in GPR #1842.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml