svnimport: Mention -r in usage summary

I added the -r option to git-svnimport some time ago, but forgot to
update the usage summary in the documentation.

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-svnimport.txt

@@ -10,10 +10,10 @@ git-svnimport - Import a SVN repository into git
 SYNOPSIS
 --------
 'git-svnimport' [ -o <branch-for-HEAD> ] [ -h ] [ -v ] [ -d | -D ]
-			[ -C <GIT_repository> ] [ -i ] [ -u ] [-l limit_rev]
-			[ -b branch_subdir ] [ -T trunk_subdir ] [ -t tag_subdir ]
-			[ -s start_chg ] [ -m ] [ -M regex ]
-			<SVN_repository_URL> [ <path> ]
+		[ -C <GIT_repository> ] [ -i ] [ -u ] [-l limit_rev]
+		[ -b branch_subdir ] [ -T trunk_subdir ] [ -t tag_subdir ]
+		[ -s start_chg ] [ -m ] [ -r ] [ -M regex ]
+		<SVN_repository_URL> [ <path> ]
 
 
 DESCRIPTION