Document -u option in git-svnimport man page

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-svnimport.txt

diff --git a/Documentation/git-svnimport.txt b/Documentation/git-svnimport.txt
index e97d15e..71aad8b 100644
--- a/Documentation/git-svnimport.txt
+++ b/Documentation/git-svnimport.txt
@@ -68,6 +68,9 @@
 	Prepend 'rX: ' to commit messages, where X is the imported
 	subversion revision.
 
+-u::
+	Replace underscores in tag names with periods.
+
 -I <ignorefile_name>::
 	Import the svn:ignore directory property to files with this
 	name in each directory. (The Subversion and GIT ignore