User's Manual: remove duplicated url at the end of Appendix B

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -4560,4 +4560,3 @@ Alternates, clone -reference, etc.
 More on recovery from repository corruption.  See:
 	http://marc.theaimsgroup.com/?l=git&m=117263864820799&w=2
 	http://marc.theaimsgroup.com/?l=git&m=117147855503798&w=2
-	http://marc.theaimsgroup.com/?l=git&m=117147855503798&w=2