patch 9.1.0676: style issues with man pages

Problem:  style issues with man pages
Solution: update man pages and test_xxd.vim, since it uses
          the xxd man page (RestorerZ)

closes: #15489

Signed-off-by: RestorerZ <restorer@mail2k.ru>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/vimdiff.man b/runtime/doc/vimdiff.man
index a053e6a..a1f05bd 100644
--- a/runtime/doc/vimdiff.man
+++ b/runtime/doc/vimdiff.man
@@ -44,4 +44,4 @@
 
 
 
-                                 2001 March 30                      VIMDIFF(1)
+                                 2021 June 13                       VIMDIFF(1)