Documentation: git-svn: fix spurious bolding that mangles the output

Documentation: git-svn: fix spurious bolding that mangles the output

Without this fix, the output looks like:

"Keep in mind that the  (asterisk) wildcard of the local ref (right of
the :) *must be the ..." -- with half the sentence spuriously bold.

This fixes the problem by simply escaping asciidoc syntax as suggested
by Jeff King <peff@peff.net>.

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