Update download links from ftp to http

[Rodney Beede]

ftp.gnu.org is slow, and it is difficult to find a mirror that is
actually up-to-date.  I tried http://ftpmirror.gnu.org/  as per
http://www.gnu.org/prep/ftp.html  but the mirror was actually
out-of-date by months.

It'd be better to just use http://ftp.gnu.org/gnu/ instead in
documentation links.

Example:

http://www.gnu.org/software/grub/manual/grub.html#Obtaining-and-Building-GRUB

ftp://ftp.gnu.org/gnu/grub  should just be  http://ftp.gnu.org/gnu/grub