manual: use current date/time for 'generated on'

Currently, the manual uses the last modification date and time of
manual.txt in the generated manual.

This is confusing, especially for long-checked-out repositories where
the top-level manual.txt has not changed since the check out. Moreover,
the manual explicitly states 'generated on', which is confusing at best.

Use the current date and time instead.

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