Be consistent in switch usage for tar

tar handles switches with and witout preceding '-', but the
documentation should be consistent nonetheless.

Signed-off-by: Henrik Austad <henrik@austad.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -1009,7 +1009,7 @@ $ git init
 If you have some initial content (say, a tarball):
 
 -------------------------------------------------
-$ tar -xzvf project.tar.gz
+$ tar xzvf project.tar.gz
 $ cd project
 $ git init
 $ git add . # include everything below ./ in the first commit: