docs/manual: get rid of legacy comment in GENDOC

Since dd935d67 (gendoc infra: use $(pkgname) instead of explicitly
passing 'manual', the GENDOC infra no longer expect any argument.

Remove the stray comment.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>