Documentation: git-svn: fix trunk/fetch svn-remote key typo

Documentation: git-svn: fix trunk/fetch svn-remote key typo

Fix the git-svn documentation svn-remote example section talking about
tags and branches by using the proper key "fetch" instead of "trunk".
Using "trunk" actually might be nice, but it doesn't currently work.

The fetch line for the trunk was also reordered to be at the top of the
list, since most people think about the trunk/tags/branches trio in that
logical order.

Signed-off-by: Wesley J. Landaker <wjl@icecavern.net>
Acked-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>