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

Documentation/git-svn.txt

@@ -693,7 +693,7 @@ listed below are allowed:
 	tags = tags/*/project-a:refs/remotes/project-a/tags/*
 ------------------------------------------------------------------------
 
-Keep in mind that the '*' (asterisk) wildcard of the local ref
+Keep in mind that the '\*' (asterisk) wildcard of the local ref
 (right of the ':') *must* be the farthest right path component;
 however the remote wildcard may be anywhere as long as it's own
 independent path component (surrounded by '/' or EOL).   This