Makefile: Fix location for the SECURITY.md file

It was renamed from SECURITY.txt -> SECURITY.md

Closes: #175
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Makefile

@@ -5,7 +5,7 @@ SITEMAP		?= python3 sitemaps.py
 URL		?= https://unit.nginx.org
 GOOGLE		?= https://www.google.com/webmasters/tools/ping?sitemap=
 BING		?= http://www.bing.com/ping?sitemap=
-UNIT_SECURITY	?= https://github.com/nginx/unit/raw/master/SECURITY.txt
+UNIT_SECURITY	?= https://github.com/nginx/unit/raw/master/SECURITY.md
 
 BUILDDIR	?= build
 DEPLOYDIR	?= deploy