Documentation/Makefile: remove extra /

As both DESTDIR and the prefix are supposed to be absolute pathnames
they can simply be concatenated without an extra / (like in the main Makefile).
The extra slash may even break installation on Windows.

[jc: adjusted an earlier workaround for this problem in the dist-doc
 target in the main Makefile as well. ]

Signed-off-by: Martin Waitz <tali@admingilde.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>