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.1 b/runtime/doc/vimdiff.1
index ddcb30f..efbb84a 100644
--- a/runtime/doc/vimdiff.1
+++ b/runtime/doc/vimdiff.1
@@ -1,4 +1,4 @@
-.TH VIMDIFF 1 "2001 March 30"
+.TH VIMDIFF 1 "2021 June 13"
 .SH NAME
 vimdiff \- edit between two and eight versions of a file with Vim and show differences
 .SH SYNOPSIS