openssh: fix a 17 year old bug by using --with-mantype=doc

Using --with-mantype=man slightly mangles ssh docs:

- See #418140 for the discussion of the part of the issue that causes
  "Ns" to be interspersed throughout the text, which dates back (and
  was reproducible) on nixpkgs from 2007. We discovered several other
  unimplemented control sequences, including -nosplit, Qq, and Mt, as
  well as the lack of spacing.
- Note the comparisons at https://github.com/numinit/ssh-man-compare.

Has been around since deb889e5f2, possibly
even earlier.

pkgs/tools/networking/openssh/common.nix

@@ -109,7 +109,7 @@ stdenv.mkDerivation (finalAttrs: {
       "--sbindir=\${out}/bin"
       "--localstatedir=/var"
       "--with-pid-dir=/run"
-      "--with-mantype=man"
+      "--with-mantype=doc"
       "--with-libedit=yes"
       "--disable-strip"
       (lib.withFeature withPAM "pam")