git-svn: trailing slash in prefix is mandatory with --branches/-b

git-svn: trailing slash in prefix is mandatory with --branches/-b

Make clear in the documentation that when using --branches/-b and
--prefix with 'init', the prefix must include a trailing slash.
This matches the actual behavior of git-svn, e.g.:

 $ git svn init -Ttrunk -treleases -bbranches --prefix xxx \
     http://svn.sacredchao.net/svn/quodlibet/
 --prefix='xxx' must have a trailing slash '/'
 $

This was noticed by R. Vanicat and reported through
 http://bugs.debian.org/429443

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>